John's Gospel, however, does not follow this schedule. Instead the last supper is eaten the night before the Passover (John 13:1) and then Jesus is executed on the "day of preparation" for the Passover meal (18:28; 19:31).
New Testament scholars have long debated which, if any, of the schedules is correct. Did Jesus die before or after the Passover meal? Was the last supper on a Wednesday or a Thursday? The apostle Paul is of little help here since although he connects the death of Jesus with Passover in 1 Corinthians 5:6-8, he makes no mention of Passover when talking about the Lord's Supper in 1 Corinthians 11:17-34.
In a recent recorded interview on the BBC Colin Humphreys of Cambridge University presents one possible solution. In a new book, The Mystery of the Last Supper, Humphreys suggests that the apparent discrepancy between the synoptic gospels and John can be explained by examining different calendars that existed at the time of Jesus. Humphreys claims that the synoptic authors used an older Egyptian calendar that was handed down from Moses. John, on the other hand, was using a newer calendar. What this means is, that, following John's new calendar chronology, Jesus did eat the Passover, but did it a day earlier, on Wednesday, using the old style calendar.
This is not an entirely new suggestion. Some have suggested that John was using the Essene calendar which was a solar calendar while Mark and the synoptics followed the more common lunar calendar. Since the solar calendar only has 364 days, some have suggested that it why there is an apparent one day discrepancy between John and the synoptics. Adherents to this suggestion include Mile Annie Jaubert (La date) and Bargil Pixner.
I commend Humphreys and others who so diligently work at interpreting the scriptures. It is difficult at times to know what to make of some of the incongruities that are so evident between the four gospels. I think that when an answer from history or archaeology is forthcoming we should use those resources to help us to better understand the gospels. But I don't think this is the case here.
A more plausible, and indeed easier, solution might be to suggest that John has purposely altered the details in his gospel for theological rather than historical reasons. By having Jesus' execution take place on the "day of preparation" John is able to more fully portray him as the Lamb of God. This is how Jesus is identified in 1:29 and by having Jesus die the day before Passover John's Jesus dies at the same time that the passover lambs are being slaughtered in preparation for the upcoming feast. Thus in John, Jesus is the ultimate expression of what it means to be the slaughtered lamb of God. This is probably also the reason why John does not have an institution of the "Lord's Supper" at the last supper. Jesus is not eating passover with them, he is about to become Passover for them.
So in the end I think that, to the degree we can know, Thursday is still our best day for identifying when Jesus ate his last supper. And I think John knew exactly what he was doing when he altered the details to fit his theology. The challenge for us in this modern age is to appreciate what John did rather than try to make his gospel fit our 21st century ideas of what it means to "write history."