According to Matthew, Mark, and Luke, the “last supper” happened on the first day of the Festival of Unleavened Bread. These gospels all equate the Festival of Unleavened Bread with the Passover, and say the last supper was the Passover meal.

But the Passover meal is eaten at the beginning of Passover, and Leviticus says the Festival of Unleavened Bread starts the day after that. So the last supper would have to be the day after the Passover meal.

And then there’s the gospel of John, which says the last supper happened before the Passover festival. After Jesus was arrested, the Passover meal was still to come. And when Jesus was brought before Pilate, Passover still hadn’t happened. Passover certainly didn’t start the day before the last supper, according to John.

Non-explanations

• There’s a verse that, in some manuscripts, has Jesus say at the beginning of the meal that he won’t eat the Passover until some future time. But other manuscripts have him say he won’t eat the Passover again until then. That seems more likely to be what it was supposed to say. It may be true that it never actually says Jesus ate that meal, but even if he didn’t, that wouldn’t change the fact that the same chapter says it was the day for the Passover meal, and that meal was the Passover meal.
• There’s also a verse that says Jesus was crucified on “Preparation Day“. Does that mean the day of the Passover meal wasn’t till the next day? No, it means he was crucified the day before the Sabbath. And even if this verse did say the last supper was the day before the Passover meal, that would just further contradict the parts that say they were the same day. It wouldn’t resolve anything.
• Does the verse in John about when Jesus was brought before Pilate also mean it was the day before the Sabbath? Probably not. That verse specifically says the day of Preparation of the Passover, not the day of preparation for the Sabbath. Even if you interpret that as the day before the Sabbath during the Passover week, it would still contradict the other verses in John that say the last supper was before the Passover festival even started, and that the Passover meal still hadn’t happened when Jesus was arrested.
• Some people say the Pharisees and the Sadducees had different days for eating the Passover meal. But the first three gospels and John disagree on when the disciples considered that day to be. John has the disciples still preparing for the festival when they eat the last supper.