It starts at sundown on Saturday...
Being Jewish and all you should know this... As it stands, History is quite evident on this issue, even if you personally don't practice it nowadays.
The 1st day of the week is Sunday, but let me tell you something you evidently didn't know when you quoted Matthew 28:1. The word Sabbbath in the text is plural!!!
The correct translation of the text is the following:
After the Sabbaths [plural], when it was growing light on the "first of Sabbaths" [day one of the week], Mary Magdalene and the other Mary came to view the grave.
So how many Sabbaths are you counting?
I count two, the reference is to the Passover and to the weekly sabbath. Thus the 1st day after the sabbaths can only be the 1st day of the week or Sunday, as we generally call it. As I said, Sunday would have started at 6 PM on Saturday according to how the hebrews reckoned their days...
Well, you have a strange Bible translation. I have three here with me. None of the three gives "sabbaths" but "sabbath." I think we are wasting our
time discussing on different Bibles. We will never get anywhere.
And for when Sunday starts it is with the sunset of Saturday. I gave that night as the second night. So, you have two days and two nights. Matthew 12:40 says three days and three nights. Where is the third day and the third night? As you can see, it is all balderdash.