Pascha is not one feast among many; it is the feast the whole year turns on. On this night the Church proclaims that Christ, having died on the Cross, has risen from the tomb, and that death itself, the last enemy, has been trampled down. Everything the Old Testament carried and everything the Gospels record is gathered into this single announcement: He is risen.
The name comes from the Hebrew Pascha, the Passover: as Israel was led out of Egypt through the sea, so Christ leads humanity out of death through His own rising. The Church keeps the feast not as the memory of a distant event but as a present reality entered again each year, the midnight service, the procession, the cry “Christ is risen!” answered by “Truly He is risen!”
The date of Pascha moves. It is reckoned from the paschalion, the ancient rule that sets it on the first Sunday after the first full moon following the spring equinox, computed on the Julian reckoning shared by the whole Orthodox Church. Because the date is calculated, not fixed, every other movable feast of the year, Palm Sunday, the Ascension, Pentecost, is measured from it.
Pascha is not a single day but a season. The feast opens a period of forty days of rejoicing, during which the paschal greeting continues and the faithful do not kneel, for it is the season of the Resurrection.