You’re confusing the Passover with the Seder. The Seder is the ceremonial supper which is consumed on the first day of Passover. The synoptic gospels say that the Last Supper was a Seder and that Jesus was therefore arrested and crucified after Passover had begun. John, however, says that the Last Supper was consumed the day before the Passover, the Day of Preparation, and that the Last Supper was not a Seder but just a last meal.