WARNING! This answer is LONG! It is 7 pages long when printed. There are 40 segments. If you want to read a few segments at a time these comments are numbered for easier reference. Now, to answer the question!
1 As "The" Sabbath, not likely. As "A" Sabbath, yes. In the Hebrew Bible, the weekly Sabbath was almost always referred to as "the Sabbath". If your Old Testament is translated from the Masoretic Hebrew text then no, Nisan 15 is not a Sabbath.
2 According to the Hebrew Scriptures (Masoretic Text), Nisan 15 was never designated as a Sabbath. There were three types of Sabbaths: 1) the weekly Sabbath, 2) the land Sabbath where the land had to lay unused every seventh year 3) the Day of Atonement (Yom Kippur), an annual Sabbath that fell in the seventh month in the Jewish calendar. There were seven annual holy convocations in the Jewish Year, and six of them forbade only servile work and were never called sabbaths in the Hebrew text. There is a reason why the Day of Atonement was called a Sabbath and the others weren't. It forbade ALL work or work of ANY kind, just like the weekly Sabbath. Regarding the Day of Atonement read in the KJV Leviticus 16:29, 23:28, 30, 31; Numbers 29:7. This was just like the weekly Sabbath. For example, refer to Exodus 20:10, 31:14,15; Leviticus 23:3, Deuteronomy 5:14; Jeremiah 17:22. Notice the Day of Atonement and the weekly Sabbath both prohibit ALL work and allow no work of ANY kind. So the Day of Atonement has the same definition of the weekly Sabbath.
3 Furthermore, almost every place in the Hebrew Scriptures where "the Sabbath" is found refers to the weekly Sabbath. So too, apparently in the Greek Scriptures (New Testament). There are a few exceptions (the land Sabbath for example). https://www.biblegateway.com/quicksearch/?qs_version=KJV&quicksearch=%22the+Sabbath%22 and https://www.biblegateway.com/quicksearch/?qs_version=KJV&quicksearch=%22the+Sabbath%22&begin=47&end=73
4 There are, in the KJV, four holy convocations in the sacred seventh month of the Jewish Year (Tishri) that are called Sabbaths but three of them come from a different Hebrew word (Shabathown) than the weekly Sabbath (Shabbath) and the Day of Atonement uses the usual word for Sabbath. Shabbathown is spelled similarly to the weekly Sabbath but it means "REST". It is even translated as REST elsewhere in the KJV, and is used sometimes with the Sabbath as in "A Sabbath of REST (Exodus 16:23, 31:15, 35:2, Leviticus 16:31, 23:3, 32; 25:4,5. Just keep in mind if it forbids ALL work or work of ANY kind it is a Sabbath. If it forbids only servile work it is not a Sabbath, according to the Hebrew Scriptures. If used in conjunction with the weekly Shabbath it is a "Shabbath Shabbathown" (Sabbath of Rest).
5 Even in the Septuagint (LXX), those three holy convocations in the seventh month are called Sabbaths in the KJV are NOT called Sabbaths. They are called ANAPAUSIS which in Greek means "REST". ANAPAUSIS is also used in the Christian Greek Scriptures (the New Testament) and it means REST there too. https://biblehub.com/greek/372.htm and https://lists.ibiblio.org/pipermail/b-greek/2005-November/036733.html
6 There are places in the KJV Bible where the new moons, sabbaths, set feasts, solemnities, solemn feasts, assemblies, and such are mentioned together.These are found in I Chronicles 23:31; II Chronicles 2:4, 8:13, 31:13; Nehemiah 10:31,33; Hosea 2:11, Lamentations 2:6, Ezekiel 44:24, 45:17. The holy convocations are distinguished from Sabbaths. See also Leviticus 23:4,37, 38.
7 After these were written they remained the same until about the second or third century BC when the Jews began celebrating the holy convocations as Sabbaths. The translation of the Septuagint carried the new view of Nisan 15 being the sabbath. Check Leviticus 23:11, 15 for this. The Jewish translators of the Septuagint took the Hebrew words “on the morrow after the sabbath” in Leviticus 23:11 and changed them to “on the morrow of the first day" (of the Feast)”.This means the first day of Unleavened bread (Nisan 15) would hitherto be celebrated as a Sabbath.
8 Hence the waving of the sheaf would always occur on Nisan 16 under the Pharisean reckoning. Since “on the morrow of the first day (of the Feast) is the antecedent for Sabbath in Leviticus 23:15 then it follows that Nisan 15 was called a Sabbath. The Sadducees in the first century AD disagreed with this view. They were known as the Torah literalists of their day and they did not call the holy convocations "Sabbaths". The Pharisees did and they controlled temple worship when Jesus was alive. So, by the time Jesus was crucified early in the first century AD Jews celebrated Nisan 15 every year as a Sabbath and waved the Omer every year on Nisan 16.
9 Josephus relates this practice in Antiquities of the Jews in Book III, Chapter 10, verse5. Read the following:
“But in the month of Xanthicus; which is by us called Nisan, and is the beginning of our year; on the fourteenth day of the Lunar month, when the sun is in Aries; for on this month it was that we were delivered from bondage under the Egyptians: the law ordained that we should every year slay that sacrifice which I before told you we slew when we came out of Egypt: and which was called the Passover. And so we do celebrate this Passover in companies, and leave nothing of what we sacrifice till the day following. The feast of unleavened bread succeeds that of the Passover, and falls on the fifteenth day of the month, and continues seven days: wherein they feed on unleavened bread. On every one of which days two bulls are killed, and one ram, and seven lambs. Now these lambs are entirely burnt, besides the kid of the goats, which is added to all the rest, for sins: for it is intended as a feast for the Priest on every one of those days. BUT ON THE SECOND DAY OF UNLEAVENED BREAD, WHICH IS THE SIXTEENTH DAY OF THE MONTH, THEY FIRST PARTAKE OF THE FRUITS OF THE EARTH: FOR BEFORE THAT DAY THEY DO NOT TOUCH THEM (Capitals mine).
10 Also, the Septuagint translated the three KJV words for sabbath in (Leviticus 23:24, 32, 39) as ANAPAUSIS which simply means rest. Concerning the Leviticus 23 annual holy convocations in the Septuagint, only the Day of Atonement is correctly called a Sabbath. Nisan 15 is called a Sabbath in the Septuagint in Leviticus 23:15. The Hebrew Bible does not call any holy convocation a Sabbath with the exception of Yom Kippur (the Day of Atonement). That's because it forbade ALL WORK and work of ANY KIND.
11 The weekly Sabbath coincided with the so-called annual Sabbath the year Jesus was crucified. The reason it was a "big" Sabbath (MEGAS in the Greek) was the holy convocation coincided with the weekly Sabbath. In the KJV it is called a "high day". In our present time, the Jews refer to the holy convocations as "high" Sabbaths but in John 19:31 the sabbath that was called a "high day" was translated from the Greek word MEGAS, which means big or great. https://biblehub.com/greek/strongs_3173.htm
12 It's important to keep in mind by the time the KJV was written (1611 AD) the Jewish people had been calling the seven annual holy convocations as "high" sabbaths for centuries so the translators took the greek word MEGAS and translated it as "high" in John 19:31. The Church of England also called their religious celebration days as "high" days. However, that's not what John wrote. See the Interlinear Greek-English New Testament by George Ricker Berry (page 411) 1981 edition. Here is how Berry translates it word for word: “The, therefore Jews, that might not remain on the cross the bodies on the sabbath, because [the] preparation it was, (for was great that sabbath)… https://tinyurl.com/3h9amtfm It wasn't a "high sabbath" but simply the weekly sabbath that was MEGALES, big, or great. That was because it was a double sabbath, with both sabbaths falling on the same day.
13 It's interesting that at least three online versions of the Martyrdom of Polycarp write that Polycarp was seized on a Friday and killed on the Great (MEGA) Sabbath. http://www.earlychristianwritings.com/martyrdompolycarp.html This Sabbath (when Polycarp died) fell in February and had no reference to a Jewish holy convocation. It was big because a big event (Polycarp's death) coincided with the weekly Sabbath. Hence, it earned the title of Great Sabbath, just like people call the Friday Jesus died on a "Good Friday."
14 However, bottom line, Nisan 15 is never called a Sabbath in the Hebrew Scriptures. The Pharisees had their own tradition and it was not what the Hebrew Scriptures taught. The so-called Nisan 15 "High Sabbath" was an invention of the Pharisees and their predecessors and the Sadducees in Jesus' day didn't buy it. They disagreed with the Pharisees on this. Nonetheless, the Pharisees dominated temple worship and after the fall of Jerusalem in 70 AD the Sadducees and their followers faded away and the Nisan 15 Sabbath prevailed upon the Jewish people.
15 One way we can know that the Sabbath that followed the crucifixion was the weekly Sabbath was the urgency that surrounded the attempt to entomb the body of Jesus before the Sabbath. That wouldn't have been necessary if the annual Sabbath was the one that was bothering them. Nisan 15 forbade only servile work, not ALL work or work of ANY kind like the weekly Sabbath does.
16 Take Luke 23:53-56 in CONTEXT. We see in Luke 23:53 the Sabbath was drawing on as Jesus' friends were entombing his body and the women saw where the body was laid. Then, according to verse 56, they returned and prepared spices and rested the Sabbath day according to the commandment. So, even though they may have finished preparing spices a little past the beginning of the Sabbath, they rested on the weekly Sabbath, that is, the Sabbath according to the commandment. So Nisan 15 fell on the day of the weekly Sabbath.
17 Luke 23:53 And he took it down, and wrapped it in linen, and laid it in a sepulcher that was hewn in stone, wherein never man before was laid. 54 And that day was the preparation, and THE SABBATH drew on. 55 And the women also, which came with him from Galilee, followed after, and beheld the sepulcher, and how his body was laid. 56 And they returned, and prepared spices and ointments, and rested THE SABBATH day according to the commandment.
18 Another way to see that Nisan 15 could not be a scriptural Sabbath is to look at Nisan 21, the holy convocation that fell on the last day of unleavened bread. If Nisan 15 was a Sabbath and it fell on a Thursday that means the following Wednesday would be Nisan 21, also a Sabbath. Now, the Jews were to count seven Sabbaths following the wave sheaf offering to arrive at Pentecost, another so-called Sabbath. If Nisan 15 were a Sabbath then Nisan 21 would have to be counted as a Sabbath too. And guess what? You would end up with something less than 50 days to Pentecost. That's because there would only be six weekly sabbaths and the so-called Nisan 21 Sabbath. However, Nisan 21 was never counted over the years as one of those seven consecutive Sabbaths. So if Nisan 21 was not reckoned as a Sabbath why would Nisan 15 be a Sabbath? A holy convocation, yes, but a Sabbath? No.
19 The fact is though is the Pharisees put their traditions above the Hebrew Scriptures and followed the Greek translation of the Hebrew. Moreover, Pentecost is also a holy convocation and is not called a Sabbath, but the day AFTER the Sabbath (Leviticus 23:15,16).
20 More: Here is a link to the use of Shabbathown in The Hebrew scriptures: https://www.blueletterbible.org/lexicon/h7677/kjv/wlc/0-1/ Here is a link to the use of Shabbath in the Hebrew Scriptures: https://tinyurl.com/26peahuu Note: The weekly Sabbath was also a holy convocation (Leviticus 23:2,3).
21 Concerning the word “preparation” in the KJV there were two preparation days in the time of Jesus. There was the weekly preparation for the Sabbath and the annual preparation for the Passover. The Passover had to be prepared for because all leaven had to be cleaned out of the houses the Jews lived in. That required cleaning and inspection.
22 The preparation for the weekly Sabbath, however, fell on Friday and it was translated in various ways in the literature of the time. For example, the capitalized words below:
23 The Didache 8:1 reads: “But as for your fasts, let them not be with the hypocrites, for they fast on the second and fifth days of the week, but do ye fast on the fourth and SIXTH days…” Kirsopp Lake’s translates the second and fifth days as Mondays and Thursdays and the fourth and sixth days as Wednesdays and Fridays.
24 Judith 8:6 reads: “and she fasted all the days of her widowhood, save the EVES OF THE SABBATHS, and the sabbaths, and the eves of the new moons, and the new moons, and the feasts and solemn days of the house of Israel.
25 Polycarp 7:1 reads: “So taking the lad with them, on the FRIDAY about the supper hour, the gendarmes and horsemen went forth with their accustomed arms, hastening as against a robber.”
26 II Maccabees 8:25-26 reads: “And they took their money that came to buy them, and pursued them far but lacking time they returned: For it was the DAY BEFORE THE SABBATH, and therefore they would no longer pursue them.
27 Antiquities of the Jews 16.6.2 reads: “and they be not obliged to go before any judge on the Sabbath day, nor on the day of the PREPARATION to it, after the ninth hour.”
28 It was late Friday when the women who came with Jesus from Galilee beheld the entombment of Jesus (Luke 23:55; Mark 15:45-47). They saw Joseph wrap Jesus' body in linen (Mark 15:46) and they would have seen Nicodemus bringing the 100 pounds of spices and wrapping the linen WITH SPICES (John 19:39-42). Why the women needed to buy and prepare spices puzzles me but the historian of "first rank" Luke said they did just that. After preparing the spices the women rested on the sabbath according to the commandment. Then, according to Mark 16:1, the women bought even more spices AFTER the Sabbath was past (Saturday at sunset). There surely would have been vendors that opened their shops on Saturday night for cases just like this. Jesus definitely didn't need it, as he was already wrapped with linen with 100 pounds of spices, and what were the women to do? Unwrap the body and re-wrap it with their own spices? The Bible doesn't say why so we can only speculate.
29 Let’s read, in context, the difference between THE SABBATH and “the first day” of the feast of Unleavened Bread. First, let’s look at the KJV, which was translated from Hebrew:
Leviticus 23 1 And the LORD spake unto Moses, saying, 2 Speak unto the children of Israel, and say unto them, Concerning the feasts of the LORD, which ye shall proclaim to be holy convocations, even these are my feasts. 3 Six days shall work be done: but the seventh day is THE SABBATH of rest, a holy convocation; ye shall do no work therein: it is THE SABBATH of the LORD in all your dwellings. 4 These are the feasts of the LORD, even holy convocations, which ye shall proclaim in their seasons. 5 In the fourteenth day of the first month at even is the LORD's Passover. 6 And on the fifteenth day of the same month is the feast of unleavened bread unto the LORD: seven days ye must eat unleavened bread. 7 In the first day ye shall have a holy convocation: ye shall do no servile work therein. 8 But ye shall offer an offering made by fire unto the LORD seven days: in the seventh day is a holy convocation: ye shall do no servile work therein. 9 And the LORD spake unto Moses, saying, 10 Speak unto the children of Israel, and say unto them, When ye be come into the land which I give unto you, and shall reap the harvest thereof, then ye shall bring a sheaf of the firstfruits of your harvest unto the priest: 11 And he shall wave the sheaf before the LORD, to be accepted for you: on the morrow after THE SABBATH the priest shall wave it.
15 And ye shall count unto you from the morrow after THE SABBATH, from the day that ye brought the sheaf of the wave offering; seven sabbaths shall be complete:
30 See how that reads, in context? The Sabbath in verses 11 and 15 have as their antecedent THE SABBATH in verse three. That’s context. Now, let’s look at this same passage in the Septuagint:
31 BRENTON”S SEPTUAGINT TRANSLATION Leviticus 23: 1And the Lord spoke to Moses, saying, 2 Speak to the children of Israel, and thou shalt say unto them, The feasts of the Lord which ye shall call holy assemblies, these are my feasts. 3 Six days shalt thou do works, but on the seventh day is THE SABBATH; a rest, a holy convocation to the Lord: thou shalt not do any work, it is a sabbath to the Lord in all your dwellings. 4 These are the feasts to the Lord, holy convocations, which ye shall call in their seasons. 5 In the first month, on the fourteenth day of the month, between the evening times is the Lord's passover. 6 And on the fifteenth day of this month is the feast of unleavened bread to the Lord; seven days shall ye eat unleavened bread. 7 And the first day shall be a holy convocation to you: ye shall do no servile work. 8 And ye shall offer whole-burnt-offerings to the Lord seven days; and the seventh day shall be a holy convocation to you: ye shall do no servile work. 9 And the Lord spoke to Moses, saying, 10 Speak to the children of Israel, and thou shalt say to them, When ye shall enter into the land which I give you, and reap the harvest of it, then shall ye bring a sheaf, the first-fruits of your harvest, to the priest; 11 and he shall lift up the sheaf before the Lord, to be accepted for you. ON THE MORROW OF THE FIRST DAY the priest shall lift it up.
32 See the change? The translators changed "on the morrow of the Sabbath" to "on the morrow of the first day". Now the first day of the feast of unleavened bread (Nisan 15) becomes the day preceding the wave sheaf, rather than the weekly sabbath. That means the wave sheaf would always end up on Nisan 16. Under the rules of the Hebrew Bible the wave sheaf would always happen the day after the weekly Sabbath. Then one would count seven sabbaths (seven weeks) to the 49th day of the fifty-day count. The day that followed the seventh Sabbath would be another holy convocation that the Jews celebrated. It's interesting to note that Nisan 21 was not counted as a Sabbath in the seven Sabbath count to Pentecost.
33 Septuagint reads further: 12And ye shall offer on the day on which ye bring the sheaf, a lamb without blemish of a year old for a whole-burnt-offering to the Lord. 13 And its meat-offering two tenth portions of fine flour mingled with oil: it is a sacrifice to the Lord, a smell of sweet savor to the Lord, and its drink-offering the fourth part of a hin of wine. 14 And ye shall not eat bread, or the new parched corn, until this same day, until ye offer the sacrifices to your God: it is a perpetual statute throughout your generations in all your dwellings. 15 And ye shall number to yourselves from the day after THE SABBATH, from the day on which ye shall offer the sheaf of the heave-offering, seven full weeks: 16 until the morrow after the last week ye shall number fifty days, and shall bring a new meat-offering to the Lord.
34 See how changing THE SABBATH to ON THE MORROW OF THE FIRST DAY changes everything? The holy convocations (KJV) were changed to “Sabbaths” by the predecessors to the Pharisees and the Pharisees in Jesus’ day clung to the Nisan 15 Sabbath. The Sadducees resisted, but they had no power since the Pharisees controlled temple worship when Jesus had his public ministry. I am just stating that according to the Hebrew text Nisan 15 was NOT a Sabbath but the LXX changed that.
35 Here is how the JPS Tanach 1917 edition translates Leviticus 23:1-15. Keep in mind that the translators of the 1917 edition of the Tanach are Jews who hold the view that the seven annual holidays are Sabbaths.
36 Leviticus 23: 1 And the LORD spoke unto Moses, saying: 2 Speak unto the children of Israel, and say unto them: The appointed seasons of the LORD, which ye shall proclaim to be holy convocations, even these are My appointed seasons. 3 Six days shall work be done; but on the seventh day is A SABBATH of solemn rest, a holy convocation; ye shall do no manner of work; it is A SABBATH unto the LORD in all your dwellings.
37 This is where the Tanach parts with the KJV. It refers to the Sabbath as "A" Sabbath and not "THE" SABBATH.
38 Leviticus 23:4 These are the appointed seasons of the LORD, even holy convocations, which ye shall proclaim in their appointed season. 5 In the first month, on the fourteenth day of the month at dusk, is the LORD’S Passover. 6 And on the fifteenth day of the same month is the feast of unleavened bread unto the LORD; seven days ye shall eat unleavened bread. 7 In the first day ye shall have a holy convocation; ye shall do no manner of servile work. 8 And ye shall bring an offering made by fire unto the LORD seven days; in the seventh day is a holy convocation; ye shall do no manner of servile work. 9 And the LORD spoke unto Moses saying: 10 Speak unto the children of Israel, and say unto them: When ye are come into the land which I give unto you, and shall reap the harvest thereof, then ye shall bring the sheaf of the first fruits of your harvest unto the priest. 11 And he shall wave the sheaf before the LORD, to be accepted for you; ON THE MORROW AFTER THE SABBATH the priest shall wave it.
39 The Tanach translates this from the Hebrew.
Leviticus 23:12 And in the day when ye wave the sheaf, ye shall offer a he-lamb without blemish of the first year for a burnt-offering unto the LORD. 13 And the meal-offering thereof shall be two tenth parts of an ephah of fine flour mingled with oil, an offering made by fire unto the LORD for a sweet savor; and the drink-offering thereof shall be of wine, the fourth part of a hin. 14 And ye shall eat neither bread, nor parched corn, nor fresh ears, until this selfsame day, until ye have brought the offering of your God; it is a statute forever throughout your generations in all your dwellings. 15 And ye shall count unto you FROM THE MORROW AFTER THE DAY OF REST, from the day that ye brought the sheaf of the waving; seven weeks shall there be complete;
40 Here, the Tanach changes from the morrow after the Sabbath to FROM THE MORROW AFTER THE DAY OF REST, hence that could be the weekly Sabbath or the holy convocation.