Actually we do know. Passover lasted an entire week. It began on what we would call Palm Sunday and ended that Saturday. That’s why there was such a large crowd of Jews in Jerusalem that week. It was Passover Week.
We have to remember that John’s Gospel was not intended to be a historical presentation of Jesus. John’s intent is to show that Jesus is the promised Messiah by offering proofs. John organizes his material according to his theme. That’s why John’s Gospel does ot fit the Synoptics. It has two very interesting things happening simultaneously. First, there is John’s attempt to prove that Jesus is the Messiah. Second, (not intended) is John’s mystical understanding of Jesus.
Unlike the other writers, John is a mystic. He presents us a theological Jesus. He’s not itnerested in the historical fact as much as he is interested in the significance of Jesus in salvation history. What was the significance of Jesus? His Messiahship.
This does not mean that John is historically wrong. Not at all. What it means is that John only uses those points of history that prove the Messiahship of Christ. Anything else about Christ, John leaves out, because by the time that he writes his Gospel, everything that can be said about Christ’s life has been said by the Synoptic writers. He’s not interested in repeating the same story.
The day of preparation would be accurate as well as Passover Day. Because in ancient Judaism, the celebration was always at the Seder, which took place at the evening meel. From John’s perspective, Jesus dies before the Passover.
He deliberately does this to point out to the fact that Jesus is truly a faithful Jew. He does the work of salvation without violating the Jewish law. He dies before the evening arrives.
Fraternally,
Br. JR, OSF