What does Mark 14:12 mean?
The Passover is a single day that commemorates how God protected the Israelites during the tenth plague He inflicted upon Egypt. When God killed the firstborns of the Egyptians, He "passed over" the Israelites who had put the blood of a lamb over their doors per God's instructions (Exodus 12). The Passover is always celebrated on 14 Nisan (Leviticus 23:4–5). Directly following the Passover is the seven-day Feast of Unleavened Bread (Leviticus 23:6–8). By the time of Christ, "Passover" and "Feast of Unleavened Bread" are used interchangeably and collectively. So, while the Passover is not technically a part of the Feast of Unleavened Bread, it is treated as such.Over the history of Israel, the Mosaic law was lost and found again a few times. Feasts were forgotten and rediscovered,and specifics were altered. At this time, Galileans celebrate Passover as God ordained: in the evening of the 14th (Deuteronomy 16:6). Judeans, however, celebrate the next afternoon. Since Jews count the day from sunset to sunset, the 14th of Nisan encompasses Jesus' Passover meal with the disciples, the betrayal in the garden of Gethsemane, the crucifixion, and the burial.
The timing of the Last Supper, the crucifixion, and the resurrection has been debated by theologians for centuries. Traditionally, Jesus is thought to have been crucified on Friday. Others say Wednesday makes more sense, since Jesus would have been in the tomb all of Thursday, Friday, and Saturday: three literal and chronological days.
Thursday, also, is a possible option. He would be buried Thursday the 14th, right before evening. The 15th—Thursday evening to Friday evening—was a holiday Sabbath: the first day of the Feast of Unleavened Bread. The 16th—Friday evening to Saturday evening—was a regular Sabbath. This would place Jesus in the tomb three days—Friday, Saturday, and part of Sunday— and three nights—Thursday, Friday, and Saturday—and explain why the women couldn't go to His tomb until Sunday.
All of these are possibilities. There are others that have more historical precedence behind them. Ultimately, of course, it doesn't matter what day of the week Jesus was crucified. What matters is that He was crucified, died, was buried, and most important He rose again.