Why did Jesus celebrate Passover on the wrong day?

The nub of the problem is this: It appears that Jesus celebrated Passover with His disciples. Certainly the Synoptic (Matthew, Mark and Luke) gospels say so but John clearly states that Jesus died at the same time as the Passover Lamb. Matt. 26: 17- 19; Mk. 14:12 - 16; Lk. 22:7 - 20; John 19:31 .

The first thing we need to realise is that the Last Supper was not on a different day to the Passover. Jewish reckoning was from sunset to sunset. This is a key to a number of apparent anomalies in the accounts. It looks like a different day to us because we reckon things differently. If we look at Genesis 1:5 we can see this right at the beginning. This is a death and resurrection pattern. First you sleep, then you awaken.

The problem then becomes one of timing. Clearly Jesus couldn't do both things at once, but, as He fulfilled the Law, was this meal a Passover? The writers of the synoptics thought it was and Jesus explicitly states that it was.

In recent years, it has been realised that there was more than one version of the calendar in use at that time.  We must also remember that Jerusalem was packed with Jews from all over the Empire for the Passover feast. The different calendars would enable a practical accommodation to get the lambs slaughtered and everybody able to celebrate the feast.  

It seems that the Pharisees, the Galileans and possibly some others celebrated the Passover on the same night as Jesus did. The Sadducees (including the priests), Judeans, and again, possibly others, followed later. John records when the High Priest would sacrifice the Lamb according to the Sadducean calendar.

I think that this is the most likely solution but it is not a definitive one. Why? Because there is not yet enough material from outside the scriptures to settle the question for most scholars. Take a wander round the Internet and you will find all sorts of solutions, some reasoned and with good evidence and others bordering on the loopy.

One thing is certain. Understanding the Jewish Passover is key to understanding the Last Supper. One interesting suggestion is that it was a Passover without a lamb. I think that this would have been mentioned in the scriptures, but it recalls Isaac's question to Abraham and with it Abraham's answer, "God will provide a Lamb". And so He did.