There is a lot to be considered, and I hope this new book on the Jewish Annotated NT is of help in this regard.
If one reads the Orthodox Jewish site, the first day of Unleavened Bread is a holy Sabbath and it occurs on the 15th day of Nisan. For this reason, all the preparations for the Passover meal needed to made before the 14th of Nisan had ended at sunset.
Basically, the Passover meal was eaten then, at the beginning of the 15th of Nisan after sunset. So the disciples of our Lord ate the Passover meal at the beginning of 15th of Nisan, the 1st day of Unleavened Bread. This day was a holy Sabbath.
In John’s gospel, it seems to be saying that the religious leaders were preparing for the Passover meal. However, the term ‘Passover’ is applied to the entire eight day feast, not only in Judaism, but in a few of the NT scriptures.
Most informed Christian and Jewish scholars would say that John’s gospel account most likely was referring to the ritual purity that was required for the** Chagigah** that was eaten towards the end of the 15th day of Nisan, which would have been the Friday of our Lord’s crucifixion.
The second day of the feast of Unleavened Bread was the day following the Sabbath of the 15th of Nisan. In Rabbinic tradition this would be the waving of the firstfruits of the spring barley harvest. This would have been on the Saturday morning, of our Lord’s Passion.
(the 16th of Nisan). This is agrees with Josephus also.
Karaite’s believe that this waving of the firstfruits should be on the morning after the **weekly **Sabbath, and not on the morning after the 1st day of Unleavened Bread as Rabbinic tradition states. Thus, for the Karaites, this would have been on the Sunday morning of our Lord’s Resurrection.(the 17th of Nisan).
So, either the 16th or the 17th was the first day of the counting of the Omer for the 50 days leading up to Pentecost, or Shavuot which is the day the ten commandments were given audibly to the Israelites from Mt. Sinai.
There were other variables for the keeping of the Passover meal, such as a month later, or on the 12th of Nisan, if the 14th of Nisan was on a Saturday. (No work was to be done from Friday sundown to Saturday sundown, therefore the preparations for the Passover meal had to be made on Thursday).
Frankly, it has taken me a long time to sort these issues out, and I may not have it correct yet.
God’s peace to all, as we prepare ourselves for this most important time of our faith.