Of course a number of New Testament scholars—the Jesus Seminar comes to mind—tend to doubt that the Gospels accurately record very much at all about Jesus, with the exception of some of his sayings. Obviously if the Gospels cannot be trusted, then we have no reason to assume that there ever was a Last Supper at all. And if there was no Last Supper, then it could not have taken place on Passover.Well, there is also the testimony of Paul in 1 Cor. 11.23-26, which most take as the earliest witness to some kind of Last Supper tradition. Of course it does not help us a lot with the question of dating, though it parallels the Gospel traditions by locating the meal on "the night that Jesus was handed over". And there are interesting Passover motifs elsewhere in the letter, especially 1 Cor. 5.7, "Christ our paschal lamb has been sacrificed" (which Klawans later mentions in another context). The letter may have been written around Passover time since Paul wishes to stay at Ephesus "until Pentecost" (16.8).
While three of the four canonical Gospels strongly suggest that the Last Supper did occur on Passover, we should not get too comfortable based on that. The three Gospels that support this view are the three synoptic Gospels—Matthew, Mark and Luke. As anyone who has studied these three Gospels knows, they are closely related. In fact, the name synoptic refers to the fact that these three texts can be studied most effectively when “seen together” (as implied in the Greek etymology of synoptic). Thus, in fact we don’t really have three independent sources here at all. What we have, rather, is one testimony (probably Mark), which was then copied twice (by Matthew and Luke).This is a common claim, and it is important to explain to newcomers that it indeed simplifies things to make it three vs. one. Matthew and Luke know and use Mark's Gospel therefore they probably derive their dating from Mark. All that is quite right. However, some caution is necessary. Eliminating Matthew and Luke as witnesses on the basis of their familiarity with Mark assumes that their only source of information was Mark. That may be the case -- and I am very much a minimum sources man, especially when it comes to the Passion -- but we should bear in mind that Matthew and Luke may sometimes have been familiar with parallel traditions. In other words, the likelihood that Matthew and Luke have Mark as a literary source does not automatically rule out the possibility of familiarity with other traditions. They are sometimes willing to modify Mark in line with alternative traditions, for example Luke in his eucharistic discourse, which has parallels with Paul, or in his resurrection account, which has appearances in Jerusalem that cohere with the presence of the leaders in Jerusalem at a later date.
It's worth adding too that John may not be an independent witness either. If John knows the Synoptics (the best evidence for which is, in my opinion, his account of the Anointing), we have to reckon with the possibility that he has simply adjusted the timing for theological reasons, with no parallel tradition about the dating.
According to John, Jesus died just when the Passover sacrifice was being offered and before the festival began at sundown . . . Any last meal—which John does not record—would have taken place the night before, or even earlier than that. But it certainly could not have been a Passover meal, for Jesus died before the holiday had formally begun.This is a minor error here -- John 13.2,4 speak of a last supper, though indeed one that is before the Passover.
The article also features a sidebar, but it does not render well in the web version -- it is practically incomprehensible. What I would recommend here is Felix Just SJ's table, The Death of Jesus in Mark vs. John, which is clear and accurate.
But what of Klawans's major conclusion, that the Last Supper was most likely not a Passover Seder? I think this may be right. I have argued in favour if a liturgical origin of the Passion narratives in the Gospels, that they were developed in the context of worshipping communities. This had the effect of fixing the dating of the narratives to the times when the communities were celebrating the events of Jesus' Passion, whether on 14th Nisan (the Quartodecimans, perhaps in continuity with some first century communities) or in a Passion vigil on the Thursday night / Friday after Passover, a custom that eventually came common but which may have had roots in the first century. The difficulty with the celebration of events on particular days is that those celebrating quickly come to think of the day of celebration as the day on which the events happened. My guess is that Jesus died at around Passover time, that this was remembered, that his death and resurrection were then celebrated around this time, but that the precise details were forgotten.
Klawans may well be right to lean towards the Johannine dating, though. Although the Synoptics clearly locate Jesus' death on the Passover (Mark 14.12, 16, 17), there are remnants of the alternative view in Mark 14.2. It is two days before the Passover (Mark 14.1) and the chief priests and scribes want to execute Jesus but they say, "Not during the festival, lest there will be a riot among the people". I suspect that this note assumes a Johannine style chronology according to which Jesus was arrested and executed on 13/14 Nisan, the day before Passover. The same underlying chronology is I think present in the idea that they were rushing to get Jesus' body buried in time for the Sabbath (Mark 15.42), an artificial idea if this is in fact Passover day, but a coherent notion if John is right that Sabbath and Passover coincided.