We know for certain that our Lord died on Friday (cf. Matt. 27:62, Mark 15:42, Luke 23:54, and John 19:31). We know it occurred in the month of Nisan (April) because it was the time of Passover. But the question remains: Why in the synoptic Gospels does Jesus celebrate the Passover on Thursday night (and, of course, given the fact that Passover fell on Friday in that year, this would be expected), but in the Gospel of John, Friday is “the day of preparation”? According to John, Passover fell on Saturday, which is why he refers to it as a “great Sabbath day” (cf. John 19:31). It was not only the Sabbath, but it was Passover as well. We still have to deal with this apparent contradiction.
According to The Navarre Study Bible, in Mark’s Gospel the Pharisees and Sadducees had a different way of celebrating feast days (51-52). The Pharisees were strict in their observance. If the fifteenth of Nisan fell on Friday, then that would be the day they celebrated the Passover. The Sadducees, on the other hand, were more liberal and had no problem with moving a feast day in certain situations. This practice is analogous to our modern practice of moving some feast days to Sunday when they actually occur during the week (as is commonly practiced with the feast of the Epiphany). It could also be likened to the bishops declaring a holy day not obligatory because of the day upon which it happens to fall. For example, if a holy day falls on a Friday, the bishops will sometimes dispense Catholics from the obligation of attending Mass on that particular holy day for that year.
What does all of this mean? When Jesus actually celebrated the Passover, he did it in the traditional way of the Pharisees. That is what we see in the synoptic Gospels. With the Pharisees, Jesus kept the Passover strictly in accord with what Moses said in Ex. 12. However, when John wrote about Christ’s passion, he does not put the emphasis on the Lord’s Supper that the synoptic Gospel writers do. In fact, he does not mention the Lord’s Supper at all. He emphasizes the crucifixion. Only in passing, as he describes the activity of the day, does John mention that it was “the day of preparation.” John was not speaking of the practice of Jesus and the apostles; he was speaking of the practice of the Sadducees, who had a large number of priests in their camp and great influence in the culture at the time. This fact explains why John calls Friday the “day of preparation” instead of Thursday. The Sadducees, who moved the Passover to Saturday, celebrated the day of preparation on Friday, rather than on Thursday as Jesus and the apostles did.
So it should not surprise us that Jesus lived liturgically in accord with the Pharisees rather than the Sadducees when it came to the Passover. After all, Jesus himself acknowledged that the Pharisees and scribes who sit “in the chair of Moses” must be obeyed. It was not that Jesus tended to side with the “more conservative” Pharisees over the Sadducees. Jesus simply followed the legitimate authority that he, as God, had established in Israel. At the same time, Jesus was quick to correct the “traditions of men” of either the Pharisees or the Sadducees in the process of bringing to the entire world the fullness of God’s revelation.
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