As a kid I was always confused as to why different Christian groups have different days for the crucifixion (some say it was Thursday, others Friday). This uncertainty runs even deeper than tradition, for even the Gospels of Mark and John narrate different dates for the crucifixion.
In this example, there have been plenty of attempts by apologists to reconcile these two conflicting dates, speculating that perhaps John was using some different way of telling time or perhaps in one of the narratives Jesus decided to celebrate the Passover one day earlier, and thus it can be called “Passover” in that narrative, even while the other narratives refer to that date as “before Passover.” However, none of these highly speculative attempts have ever been taken seriously, and today, the view of mainstream New Testament scholars remains that:
“John differs from the Synoptic Gospels also in the date which he gives for the crucifixion:
According to Mark the last supper was a Passover meal; that is, it was eaten in the early hours of Nisan 15; the arrest and trial took place in the same night and in the course of the next (solar) day Jesus was crucified. All these events took place on Nisan 15. “
-According to John the crucifixion happened on Nisan 14, the day before the Passover; the last supper must have been eaten the preceding evening. Thus the events are set a day earlier than in Mark, and the last supper is no longer the Paschal meal; Jesus died at the time when the Passover sacrifices were being killed in the Temple. Here again is a real contradiction; it seems impossible to reconcile the dates.”
– C. K. Barrett, President of the Society for New Testament Studies (1973)