Bart Ehrman destroys Christianity in under 12 minutes.

[Angrboda]

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.

How Do We Explain the Passover "Discrepancy"?

This Catholic Answers article seems to point to the discrepancy being real. But of course Drich knows better.