Passover was the Israelite holiday on which Christ was tried and crucified. It was at the Passover meal, often referred to as Christ’s Last Supper, that Jesus gave new meaning to a couple emblems of that meal which became a weekly Christian meal—the Lord’s Supper. Because the genesis of communion stemmed from the Passover meal, Christians infused it with meaning taken from Passover. Paul, in the earliest dated source, referred to Jesus as our Passover lamb (1 Corinthians 5:6–8). John the Apostle developed this idea in his corpus of literature (John 1:20). In his gospel account, the Passover provides a timeline for the ministry of Jesus, and it indicates that Jesus was crucified at the very moment when the Passover lamb was sacrificed. In Revelation, the lamb is representative of Jesus and His work.
By the latter part of the second century, Melito of Sardis preached On the Pasch wherein he celebrated the theme of Christ as our Passover lamb sacrificed once-for-all. In his homily, Melito explained the earliest liturgical structure of the Christian Pasch (Passover): it was celebrated during Passover from sunset until midnight. It included fasting, Scripture reading focused on Exodus 12, chanting, and partaking of the Lord’s Supper during which a homily was preached about Pasch.
By the turn of the third century, Tertullian noted how Pasch was preferable as a season of baptism: “The Passover affords a more than usually solemn day for baptism; when, withal, the Lord’s passion, in which we are baptized, was completed” (On Baptism 19). After this, Tertullian notes, a week of instruction and celebration ensued. By the third century, Holy Week, Easter, and Easter Week celebrations were established. At the end of the fourth century, the Forty Days (later called “Lent”) developed as a time during which instruction and preparation for baptism were undertaken.
While the earliest Christian sources we have, the New Testament, do not attest to any celebration per se, the importance of this period was noted in Christian theology at least by recognizing Christ as the Passover lamb slain for the sins of the world. It would only seem natural that a celebration or period of observance would follow. Nevertheless, because Passover is still kept by Israelites the world over, we have a rather sure idea of the time when Christ was crucified.