• Mark 14:12 . . And the first day of Unleavened Bread, when they killed the Passover, his disciples said unto him: Where wilt thou that we go and prepare that thou mayest eat the Passover?
That passage can be a bit misleading for folks unaware that the seven-day feast of Unleavened Bread (a.k.a. Passover) consists of a total of eight days.
The "first day" is when all the lambs are slaughtered and roasted with fire, ready to eat by sunset; which is when the seven-day part of the feast kicks off seeing as how the Jews' holy days begin and end with sunset.
In other words: the "first day" isn't a holy day. In point of fact it is a very busy day because of all the preparations that have to be made for dinner that night; which includes a witch hunt (so to speak) for any and all leavened bread in the Jews' homes.
• Matt 26:3 . .Then the chief priests and the elders of the people assembled in the palace of the high priest, whose name was Caiaphas, and they plotted to arrest Jesus in some sly way and kill him. But not during the Feast-- they said --or there may be a riot among the people.
They failed to accomplish Jesus' death apart from the Feast.
The fixings for Jesus' Passover were prepared on the day leading up to the night of his arrest. The Jews prepared for their Passover on the day of his crucifixion. In other words: Jesus' Passover was prepared a day ahead of theirs.
He being a prophet in direct contact with God would know the correct date of the "first day" of the Feast of Unleavened Bread, whereas the Jews' calculations were vulnerable to human error. Consequently the Jews were unaware that their religious calendar was running behind that year.
So, as it turned out; Jesus was crucified on what the Jews believed was the day of preparation, but in reality was one of the seven days of the Feast.
* The Jews' mistake worked to Jesus' advantage. Had their calendar been correct, then Jesus would've lost an opportunity to share one last sacred event with his men; something he really wanted to do.
"Then, at the proper time, Jesus and the twelve apostles sat down together at the table. Jesus said: I have looked forward to this hour with deep longing, anxious to eat this Passover meal with you before my suffering begins." (Luke 22:14-15)