The feasts of Holy Week—Holy Thursday, Good Friday, Holy Saturday, and Easter—are fulfillments of the three spring Jewish feasts with which they originally coincided. We refer to the three great liturgies of Holy Week—the Mass of the Lord’s Supper on Holy Thursday evening, Good Friday services, and the Easter Vigil—as the Holy Triduum. In the year of Christ’s death, the Jewish Passover on 14 Nisan would have begun at sundown on Thursday, the same evening as the celebration of the Last Supper and been completed at sundown on Good Friday, the day of His death.
What we call Holy Saturday, the day that Christ lay in the tomb, a day without its own liturgy, coincides with the first day of the feast of Unleavened Bread on 15 Nisan, which is closely associated with Passover. Leaven, a sign of sin, is cleansed from all Jewish homes on the eve of Passover. And the following seven days of Unleavened Bread, called in Deuteronomy 16:3 “the bread of affliction,” are observed, beginning the day after Passover, as a memorial of the flight from Egypt and as a reminder to each Jew “of what the Lord did for me when I came out of Egypt” (Ex 13:8).
The feast of First Fruits, which coincided that year with the day of Christ’s Resurrection and commemorated the first sheaf of the barley harvest, began after sundown of 15 Nisan (according to Jewish reckoning the beginning of 16 Nisan). This feast, involving a ceremonial cutting and offering of the first fruit of this harvest, asks God’s blessing in anticipation of the full harvest in the spring at the Feast of Weeks or Pentecost.
Read more at St Paul Center for Biblical Theology