“Now on the first day of Unleavened Bread.” The commentators recognize that this phrase involves an idiomatic understanding of the feast of Unleavened Bread, because technically the feast of Unleavened Bread began the evening after the Passover Lamb was sacrificed (Exod. 12:15-20; Lev. 23:6), and this event in Matthew—and the Last Supper associated with it—occurred before that time.
Different scholars postulate different possibilities for the meaning of this phrase because it is not literal, but it is not difficult to understand what is being said here. Technically the first day of the Feast of Unleavened Bread was on the 15th of Nisan, which started at sunset after 14th of Nisan, when the Passover Lamb was sacrificed. However, as Lenski correctly observes, the “first day” “originally designated the celebration of the afternoon and evening of the 14th of Nissan (the eating of the Paschal Lamb) and [then] naturally came to be used by both Jewish and Greek writers also for the entire week of the celebration that followed” (R. C. H. Lenski, The Interpretation of St. Mark’s Gospel, note on Mark 14:12).
So, from the way people commonly thought, the Feast of Unleavened Bread originally and technically did not start until sunset ending the 14th of Nisan and starting the 15th of Nisan (the Jewish day started at sunset). Then in time the “first day” of the Feast of Unleavened bread included the afternoon of the 14th of Nissan when the Passover Lamb was killed, then eventually the saying, the “first day of Unleavened Bread,” came to occasionally be used to refer to the whole week, and that is the way it is used here in Mark. Similarly, just as the Passover gets swept up in common language into the Feast of Unleavened Bread, the Feast of Unleavened Bread is sometimes called the “Passover” (cp. Luke 22:1). In the United States a similar thing has occurred with Christmas, and it would not be uncommon for someone to see Christmas lights and trees and exclaim “It’s Christmas!” when technically Christmas (Dec. 25th) was still even weeks away.
The season of Passover and Unleavened Bread took some preparation, for example, the Passover lamb was selected on the tenth day of Nisan, and that was a couple of days before the disciples ate the Last Supper especially if Jesus was arrested in Gethsemane after dark on our Monday night, which would have been Nisan 13 to the Jews. Also, although people did not technically have to remove the leaven from their houses until the 15th of Nisan, the first day of the Feast of Unleavened Bread (Exod. 12:15), we know that later in history people started making plans to remove the leaven days in advance, and there is little reason that could not have happened in Christ’s time as well.
Although we cannot pin down the exact meaning of the idiom here in Matthew, or the exact day it referred to, it is clear that Matthew, who was an observant Jew, knew only too well that technically the Feast of Unleavened Bread occurred the night after the Passover was killed, but that did not stop him from writing what he did in his Gospel. This tells us that Matthew and his immediate audience knew something that we modern interpreters do not know; but likely it had to do with the feast of Unleavened Bread being idiomatically used for that general season.
[For more information on Jesus being in the grave for three days and three nights—from Wednesday Nisan 14 to Saturday Nisan 17, see commentary on Matthew 12:40, “three days and three nights.” For more information on the events from Jesus’ arrest through his resurrection appearances, see commentary on John 18:13 and 19:14. For more information on Nicodemus and that he came after Joseph of Arimathea left the tomb, see commentary on John 19:40].