There is considerable debate over the date of the Last Supper which centers on differences between the Synoptic Gospels (Matthew, Mark, and Luke) and the Gospel of John.
In a nutshell, the Synoptic Gospels suggest that the Last Supper was a genuine Passover meal, held on the evening before Jesus' crucifixion. In contrast, the Gospel of John implies that the Last Supper took place before the Passover festival.
Several hypotheses have been proposed for how to solve this dilemma.
What do I think? Well, below is a summary of my argument about the Last Supper from my forthcoming book on the historical Jesus.