What was the last supper about? Was it a Seder? Let's find out.
The Synoptic Gospels state that Jesus arranged to eat his last meal, commonly called The Last Supper, with the twelve apostles (Matthew 26:17-20, Mark 14:12-17, Luke 22:7-14). They present this last meal as a seder although no Gospel ever mentions that a lamb (Exodus 12:26-27) or bitter herbs (Exodus 12:8, Numbers 9:11) was part of the meal. The earliest New Testament reference to this meal is by Paul (1 Corinthians 11:23-25). He makes no connection between the last meal and the seder. The Synoptic Gospels describe only the Last Supper in a Passover context, never giving any indication in their respective accounts of the arrest, trials, crucifixion, death, and burial of Jesus that the day following this meal is Passover.
In the context of an alleged Passover prisoner release custom the phrase "at a/the feast" appears (Matthew 27:15, Mark 15:6, and the interpolation Luke 23:17) but gives no indication of whether this was to take place before or during the festival. The phrase may refer to any day during the festival and does not specify any one day in particular. The author of Mark, the earliest of the Synoptic Gospels, turned the meal into a seder with the comment: "And on the first day of Unleavened Bread, when the Passover [lamb] was being sacrificed, his disciples said to him [Jesus], 'Where do you want us to go and prepare for you to eat the Passover?'" (Mark 14:12). From the sequence of events presented in the Gospels it would appear that the last meal occurred on a Thursday evening prior to Jesus' execution the NEXT afternoon (that is, either on 14 Nisan or 15 Nisan).
While the Synoptic Gospels portray the Last Supper as a Passover seder (15 Nisan), John's Gospel dates the Last Supper a day earlier (14 Nisan), making it a regular meal. The author of John has no intention of equating the Last Supper with the seder. At the beginning of his Gospel, he has John the Baptist identify Jesus as the "lamb of God" (John 1:29). He now depicts Jesus as dying on the afternoon before the beginning of the Passover festival, at the same time as the lambs were being sacrificed in the Temple: "Now it was the day of preparation for the Passover; it was about the sixth hour (John 19:14). Apparently, either the Synoptic Gospels wanted to turn Jesus' last meal into a seder or John wanted to link Jesus' death with the slaughter of the paschal lambs.
In any case, although all the Gospels agree that Jesus' death occurred on a Friday, neither date or only one date can be historically correct. From the Gospels respective accounts we cannot know when Jesus died--the afternoon before Passover or the afternoon of the first day of the festival. It is also possible that Jesus' execution may have taken place on another day (Mark 14:1-2). Acceptance of one version over the other does not solve the issue of the historicity of either one. That is, there is no Question of the historical existence of conflicting traditions but there is a Question as to the historicity of the events they portray.