Here's the problem concerning the singular/plural distinction: I found an online Peshitta forum post by Paul Younan (who prepared a scholarly Peshitta text) that mentions that there was no way to distinguish between the singular and plural in Aramaic until at least the 6th century. He states:
Notice the only difference between the two is the Syame (plural) dots
above the letter Beth....
It's really an annoying feature of Aramaic. I can't believe that it
took till the 6th-7th centuries for someone to come up with this
brilliant idea of somehow differentiating between the singular and the
plural.
Or, not having vowels. Ancient Akkadian had 'em - no problem with
singular/plural, either.
It's strange - as time went on, rather than progressing - these
writings systems digressed.
It's not just translators who got confused. The only way to tell was
from memory or context - and sometimes both memory and context were
lacking.
I will go through each verse individually and answer the questions, but before I do allow me to acknowledge my morphology sources: The first is A Compendious Syriac Dictionary, a Syriac dictionary founded upon the thesaurus syriacus of R. Payne Smith. According to this source (p. 558), the word for Sabbath / Saturday can also mean "week" depending on the context and other words used with it:
I have also cross-referenced the morphology using George A. Kiraz's Analytical Lexicon of the Syriac New Testament : Based on the SEDRA 3 Database of George Anton Kiraz. (Bellingham, WA: Logos Research Systems, Inc., 2003), although only the gloss is given by Kiraz (Sabbath; yet it is consistently translated by scholars as 'week' into English as noted in each case below).
So with all of that said, it is possible that some or all of this post is wrong, because almost 400 years transpired where no one knew which word was singular or plural except based on handed-down tradition. Younan believes that the Gospels were originally written in Aramaic and then translated into Greek, and commenting on the singular/plural problem he states,
Obviously, if as we contend the Greeks translated the Aramaic Gospels
into Greek at a very early stage (perhaps even done by the Apostles
themselves or their immediate successors) - then it would make sense
that they would have made mistakes because of the singular/plural
problem, prior to the invention of the Syame dots which were not
around until after the 6th century....
To avoid getting into the 'chicken and egg' debate concerning the translation of the New Testament (which came first, the Aramaic or the Greek?), I will avoid any comparisons to the Greek text in this response and treat the Syriac Peshitta as an independent text.
So it appears that context will be our best (and potentially our only) friend when it comes to discerning whether the word should be translated as 'Sabbath' or 'week.'
- Matthew 28:1 - The same root word is used here both for 'Sabbath' and 'week;' context determines the different interpretation of each. In both cases this word is a feminine singular emphatic noun, although there is a slight spelling difference in the text between the two - the second instance (translated 'week') appears to be using a tet instead of a taw, but each goes to the same lexicon entry. However, morphologically it is the same root word and I have not found any scholarly literature that supports a distinction based on spelling, but rather only based on context. But when it is translated as 'week,' this spelling difference appears to be used consistently. Dr. Frank Hardy gives a detailed explanation of this verse and why "first day of the week" is the appropriate translation of this word in this passage (rather than "on the first Sabbath"), on the basis of the definite article given in the Syriac. This explanation also applies to most of the verses in this list.
- Mark 16:2 - The same word is used here; its context dictates translation as "week." It is also a feminine singular emphatic noun. From now on, I will simply give the best contextual translation and decline the word like so: FSEN = feminine singular emphatic noun.
- Luke 24:1 - Same word, best translated as "week," FSEN.
- John 20:1 - Same word, best translated as "week," FSEN.
- John 20:19 - Same word, best translated as "week," FSEN.
- Acts 20:7 - Same word, best translated as "week," FSEN.
- 1 Corinthians 16:7 - Neither 'Sabbath' nor 'week' are used in this passage. I presume you meant to ask about 1 Corinthians 16:2, in which case the same word is used, is best translated as "week," and is a feminine singular emphatic noun.
It should be noted that every scholarly English translation of the Peshitta that I have referenced (Etheridge, Murdock, and Lamsa) translates the word as I have indicated in the above list. This is my basis for asserting what the best translation is in each case.
There is another verse that will help shed some light on this as well: Luke 18:12, which the KJV translates,
"I fast twice in the week, I give tithes of all that I possess."
The same word that can mean "Sabbath" or "week" is used in the Syriac Peshitta, again here as a feminine singular emphatic noun. However, the taw is used (which elsewhere this spelling change is consistent with 'Sabbath,' but again, I have found no scholarly literature mentioning this spelling difference as having anything to do with the translation - this is only my observation when reading the Peshitta text). Discussing this passage, Joe Viel says,
Fasting "twice each Sabbath" makes no sense. How do you fast twice a
day? Do you eat non-stop except for two breaks????? There was a Jewish
tradition of fasting twice a week, on Mondays and Thursday, that is
well documented in the Talmud as well as other places, including some
early Christian writings.
UPDATE: I asked a scholar trained in Syriac about this.
I hope this answer gives interested parties a good 'start' and a good introduction to the question. I am not sure anyone has explored this much further as of yet (research opportunity!), or if it is even possible to pursue it much further. Please post any supported revisions or other notable information in the comments (or edit this answer if warranted).