Hi Rstrats: I remember seeing your OP before, I think on another forum where a poster (in agreement with me) by the name of John 8:32 said to the effect that Christ himself defined the day he was using, i.e. are there not 12 hours in a day? Being into sundials, I took that as a phase we gnomonists would understand given that ALL days back then, being they were long or short had 12 hours (being divided equally). As a matter of fact, that's the only biblical mention of 12 hours in a day (although other passages mention numerical hours). So that is confirmation in such jargon that he was defining a day from sunrise to sunset and night would thus be sunset to sunrise. So in Matthew's passage (given that definition) when he said 3 days and 3 nights, then it ceases to be an idiom and means a full 72 hours. If we look at the story of Jonah, we see the sea turbulence in the ocean Jonah was thrown into (likened unto Jesus dying during the crucifixion) until he is swallowed by a great fish (likened unto the tomb (Matthew 12:40). So I took this to mean 3 days and 3 nights in the tomb, which was almost immediately after his physical death anyway (thus, I won't debate someone who considers the start of his death about 3PM as the beginning of the 72 hours or a tad later at 6PM about when he was placed in the tomb-- it's essentially the same time of the day, i.e. the evening.) I lean toward the latter time and I'll explain shortly.
That all said, there was the day of Firstfruits, representing Christ's resurrection and ascension. This happened the "morrow after the Sabbath". When cutting the wave sheaf the question would be asked, "Did the sun set yet?" This foreshadowed the time of Christ's resurrection. So when was this? Late Sabbath as Firstfruits was dawning, thus Sunday. Now, we can make a calendar. Christ died late the 14th of Nisan to fulfill the Passover, and unleavened Bread followed immediately afterwards Nisan 15 -- this was a Sabbath High Day. So if he died late on the 14th, 3 days and 3 nights later is late on the 17th as the day ended (i.e. all of the 15th, 16th and 17th in the tomb). We know that the 17th was a Sabbath (Saturday) and Sunday the Firstfruits was coming on. That makes Sunday the 18th of Nisan. Now, look on any calendar where Sunday happens to fall on the 18th and look what day of the week the 14th falls on -- Wednesday.
So we have Passover crucifixion Wednesday, A Sabbath on Thursday, Friday and then the Sabbath Saturday, and that night as Sunday dawned they were cutting down the wave sheaf as Christ was raising. Take note that 3 days and 3 nights means Christ would rise the same time of the day as he was entombed (or died). That's late afternoon as the sun was setting. So, since Christians today use the Gregorian calendar, for sure Christ arose on the Sabbath day as Firstfruits was dawning.