During Holy Week, Christians recall the events leading up to Jesus’ death by crucifixion and, according to their faith, his Resurrection. The week includes five days of special significance. The first is Palm Sunday, which commemorates Jesus’ humble entry (on a donkey) into Jerusalem to observe Passover. According to the Gospel account, he was greeted by crowds of people who spread their cloaks and laid palm leaves in his path and proclaimed him the Son of David (Matthew 21:5). In many Christian churches Palm Sunday is celebrated with a blessing and procession of palms. Maundy Thursday marks Jesus’ institution at the Last Supper of the Eucharist, thereafter a central element of Christian worship. In Roman Catholicism, Maundy Thursday is accompanied by the pope’s washing of the feet of 12 humble or poor persons, in imitation of Jesus’ washing of the feet of his 12 disciples at the Last Supper. Good Friday commemorates Jesus’ suffering and death on the cross; it is traditionally a day of sorrow, penance, and fasting. Holy Saturday, also called Easter Vigil, is the traditional end of Lent. Easter Sunday is the celebration of Jesus’ Resurrection, according to the Gospels, on the third day after his crucifixion. The modern observance of Easter, like that of Christmas, has become associated with various folk traditions that have little connection with the religious celebration; they include the Easter lamb, the Easter rabbit, and the painting of Easter eggs.
From: Encyclopedia Britannica
in the Christian Church, the week between Palm Sunday and Easter, observed with special solemnity as a time of devotion to the passion of Jesus Christ. In the Greek and Roman liturgical books it is called the Great Week because great deeds were done by God during this week. The name Holy Week was used in the 4th century by Athanasius, bishop of Alexandria, and Epiphanius, bishop of Constantia. Originally only Friday and Saturday were observed as holy days; later Wednesday was added as the day on which Judas plotted to betray Jesus, and by the beginning of the 3rd century the other days of the week had been added. The pre-Nicene Church concentrated its attention on the celebration of one great feast, the Christian Passover, on the night between Saturday and Easter Sunday morning. By the later 4th century the practice had begun of separating the various events and commemorating them on the days of the week on which they occurred: Judas’ betrayal and the institution of the Eucharist on Maundy Thursday; the passion and death of Christ on Good Friday; his burial on Saturday; and his Resurrection on Easter Sunday.
The Holy Week observances in the Roman missal were revised according to the decree Maxima Redemptoris (Nov. 16, 1955) to restore the services to the time of day corresponding to that of the events discussed in Scripture.