“Some have pointed out that Mark also indicates that Jesus died on a day that is called ‘the Day of Preparation’ (Mark 15:42). That is absolutely true — but what these readers fail to notice is that Mark tells us what he means by this phrase: it is the Day of Preparation ‘for the Sabbath’ (not the Day of Preparation for the Passover). In other words, in Mark, this is not the day before the Passover meal was eaten but the day before the Sabbath; it is called the day of ‘preparation’ because one had to prepare the meals for Saturday on Friday afternoon.
“…in Mark, Jesus eats the Passover meal (Thursday night) and is crucified the following morning. In John, Jesus does not eat the Passover meal but is crucified on the day before the Passover meal was to be eaten. Moreover, in Mark, Jesus is nailed to the cross at nine in the morning; in John, he is not condemned until noon, and then he is taken out and crucified….
“…I will point out a significant feature of John’s Gospel – the last of our Gospels to be written, probably some twenty-five years or so after Mark’s. John is the only Gospel that indicates that Jesus is ‘the lamb of God that takes away the sins of the world.’ This is declared by John the Baptist at the very beginning of the narrative (John 1:29) and again six verses later (John 1:35). Why, then, did John – our latest Gospel – change the day and time when Jesus died? It may be because in John’s Gospel, Jesus is the Passover Lamb, whose sacrifice brings salvation from sins. Exactly like the Passover Lamb, Jesus has to die on the day (the Day of Preparation) and the time (sometime after noon), when the Passover lambs were being slaughtered in the Temple.
“In other words, John has changed a historical datum in order to make a theological point: Jesus is the sacrificial lamb. And to convey this theological point, John has had to create a discrepancy between his account and the others.”
– Bart Ehrman, from his 2009 book, Jesus, Interrupted: Revealing the Hidden Contradictions in the Bible (And Why We Don’t Know About Them)