Because God is revealing to you that his calendar is based off the lunar phases and not the solar phases. Psalms 104:19 "he (God) appointed the MOON for MOEDIM". God gave you specific dates so that you could prove his calendar has never changed. Most bible scholars falsely claim that during the pre-flood era, there were exactly 30 days per month and 360 days per year. This is not true and easily proven false by Noah's flood account. Let me show you, and prove Gods calendar is based of the lunar phases.
Gen. 7:11 - The rain started in the 2nd month, 17th day.
Gen. 7:12 - It rained 40 days and 40 nights.
Gen. 7:24 - Then, the waters "prevailed" 150 days.
Gen. 8:3 - Then, the waters "abated" 150 days.
Gen. 8:13 - Waters dried on the 1st day of the new year.
These facts reveal that between the 17th day of the 2nd month and the last day of that year, totaled 340 days. But then add the first 16 days of the 2nd month that weren't counted, and then add the 29 days in the first month (Hebrew months had 29 or 30 days per month), and you get a total of 385 days.
Now ask yourself, are there 385 days in a single year, in your pagan solar calendar? Nope. I just showed you how there were 385 days in a year during the life of Noah, this proves a 13th month was present during the times of Noah. This also proves that it is impossible for every month to be exactly 30 days long. The ancient Hebrew calendar had either 354 days per year (totaling 12 months) or 384-385 days per year (totaling 13 months).
The chronological facts that are recorded in the account in the book of Genesis clearly establish a period of 385 days in the year of the Noachian Flood. This year length is significant because it is the exact number of days required for an excessive leap year in the Hebrew Calendar. This remarkable account of the Flood, which God inspired to be recorded in His Word, is indisputable evidence that the Hebrew Calendar bears His stamp of approval. There can be no doubt that the calculations of the Hebrew Calendar have been the basis of God’s true calendar from the beginning.
Now back to my statement about our calendar being dictated by the MOON. God commanded in Psalms 104:19 "I appointed the MOON for MOEDIM". So what are Gods MOEDIM that must be appointed according to the MOON (not the SUN)? Leviticus chapter 23 lists a total of 8 MOEDIM that are to be appointed according to the MOON. The 7th day Sabbath is the very first MOEDIM listed in Lev.23 to be appointed by the moon, not the solar cycle like pagan Rome enforced in the 4th century under emperor Constantine, and accepted by the Roman Catholic Church.
Now think back to Daniel 7:25, when God warned us how we know if we have been deceived by the Devil. Daniel 7:25 "The BEAST will think to CHANGE my TIMES and LAWS". I just showed you how the Roman Catholic church, paired with Constantine, changed Gods "TIMES". They switched Gods luni-solar calendar to a "solar only" calendar. This was only possible when Emperor Constantine declared under penalty of death, that no one could obey the "Jewish" holy days. This eliminated the "New Moon Day". Which starts the beginning of all of Gods months. Ezekiel 46:1 tells us there are 3 types of days (New moon day, 6 work days and then a 7th day Sabbath). Rome enforced the elimination of the new moon day, meaning the 6 work days started a day earlier than God scheduled, making it so the 7th day Sabbath was now to be calculated according to a 7 day weekly cycle, according to the sun, not moon.
Be aware and do not learn the ways of the heathen. Even Zechariah 14:16 promises us that when Jesus returns, all nations must obey the feast days (according to the moon/Psalms 104:19) and Zechariah 14:21 says that when Jesus returns to earth we all must obey the new moon days and Sabbaths AND we will still bring sacrifices to him.
The word "Christian" was only used 3 times in the bible. We are told a Christian is someone who "obeys and teaches everything that Jesus did and taught". Jesus taught the feast days, the MOEDIM, according to the MOON, just like his father commanded in Psalms 104:19. It wasn't until about 300 years AFTER the death and resurrection of Jesus that any country in this world switched to a SOLAR BASED CALENDAR! Don't be fooled.
During the Exodus from Egypt, we are told the 15th day of the 1st, 2nd AND 3rd month were Sabbaths. This is impossible on a solar, 7 day weekly cycle calendar. Wake up friends, God does not calculate his Sabbaths according to the sun cycle like pagan sungod worshipers do. God commanded his 7th day Sabbath to be appointed according to the moon, just like the Passover, unleavened bread, first fruits, weeks, trumpets, atonement, and tabernacles. "I appointed the MOON for MOEDIM". The Sabbath is literally defined as Gods very first MOEDIM, which means it MUST be appointed by the MOON.