Here is an excerpt:
One of the more famous and most discussed differences on chronology in the gospels deals with the timing of the Last Supper and the Crucifixion. Here is a difference often trumpeted forth as a clear error between the Synoptics and John. This is the very issue raised by Daniel Kirk in his contribution to Pete Enn’s “Aha moments” series. However, things are not so simple.
Here is the problem in a nutshell. In Mark 14:12 we are told the supper took place on the first day of unleavened bread when they sacrificed the Passover lamb also Luke 22:15 goes in this direction referring to this Passover. Part of what is taking place here is that the Passover 14 Nisan and unleavened bread 15 Nisan and following for a week were celebrated one right after the other, so that both names became attached to the feast Josephus, Jewish War 5.99, speaks of the feast of unleavened bread on 14 Nisan. The very way in which these dates and feasts are handled shows proximate dating could be used. As a popular designation, one could refer to the whole period as either Passover or Unleavened Bread. The problem comes in that John 19:14 and 31 have Jesus crucified on the Passover preparation day which appears to be the day after the meal the Synoptics portray as the Passover meal. This is a complex problem as a detailed study by Howard Marshall shows.