If the Israelites had kept Passover on the wrong day, they would have died! If we accept error without proving it, we set ourselves up to accept other errors.
The Night Much to be Observed is a memorial of the covenant with Abraham, and God's watchfulness in delivering ancient Israel as well as spiritual Israel.
Israel was to keep the Night to Be Much Observed as a night of watching—of watchful vigil—to commemorate the reason they were able to leave Egypt so easily.
Confusion over time of Passover, the wavesheaf offering, and Pentecost results from making assumptions unwarranted by clear scriptural evidence.
Does it matter when we keep the Passover as long as we keep it? The logistics of Israel leaving Egypt prove that Passover should be kept on the 14th, not the 15th.
The word 'selfsame' refers to a specific commemorative date. The selfsame day is a signal that God is faithfully in control of time over multiple centuries.
The context of Deuteronomy 16:1-3 indicates the focus of these verses is on the Night to be Observed and the Days of Unleavened Bread rather than the Passover.
Passover takes place at twilight as the 14th of Abib begins. Unleavened Bread begins 24 hours later on the 15th of Abib. The Passover is a preparation day.
The proponents of a 15th Passover discount clear scriptural details and instead speculate. One cannot build doctrines on implication, distortion, and traditions.
The term 'selfsame day' refers to the covenant God made with Abraham 430 years before the Exodus, which occurred on the day after the Passover.
The timing of Christ's crucifixion does not coincide with the Passover, but instead lines up with the covenant God made with Abraham, marking a major fulfillment.
The temple Passover commanded by Hezekiah was a very unusual circumstance in which the king centralized worship to keep Baalism from defiling the Passover.