Easter (also called “Pascha” or some variant by most non-English speaking Christians) is the feast celebrating Jesus’ resurrection from the dead, and is the greatest and oldest feast of the Church. Even the term “Pascha” is borrowed from the Jewish word for “Passover,” and Easter is calculated based on the lunar calendar (all other feasts are on the solar calendar). These facts show the ancient, probably Apostolic, origins of Easter. We even possess a baptismal liturgy of Easter dating to the mid-third century. Traditionally, the Pascha celebration began with a lengthy vigil, the “mother of all vigils” according to St. Augustine. The whole history of salvation is retold during the vigil, through scripture and liturgy. At the Easter Vigil (in the West) three traditions developed: the baptism of new converts, lighting of the paschal candle, and the blessing of the new fire (taken from the Jewish blessing of the lamp on the eve of the Sabbath). The new fire is often processed into the Church to light the Paschal candle. Eucharist is then celebrated in the morning hours, being also the first Eucharist of new converts. In general, Roman Catholic, Orthodox, and Anglican Vigil services consist of variants of this ancient model. The West also celebrates the octave of Easter. These eight days are all solemnities in the Western liturgical calendar. Actually, these days even take precedence over other solemnities that can fall within the Octave of Easter, including the Annunciation.
Easter follows Holy Week, the end of the season of Lent. The Evening Prayer (Vespers) of Easter ends the Paschal Triduum, the three-day period that began on the evening of Holy Thursday. The Triduum contains the heart of the Christian faith: Jesus’ death and resurrection. Easter is not just a day, but an entire fifty-day season, called Eastertide, marked by joyful festivities and liturgical fullness. You might hear “Christ is Risen!” and “Alleluia!” frequently during the Easter season, because we are joyfully celebrating Christ’s bodily resurrection. The Feast of the Ascension falls within Easter season. The 50-day season of Easter runs up to the Feast of Pentecost.
Of note, Western and Orthodox celebrations of Easter (Pascha) vary in certain ways. Usually Orthodox and Western Christians celebrate Easter on two different Sundays. The reason is that Orthodox churches still base their calculation of Easter’s day on the Julian calendar, whereas Western churches follow the Gregorian calendar. In order to keep the date of Easter on a Sunday, the date changes yearly based on the Paschal full moon. The possible date range for Western Easter day is March 21st-April 25th. So what is the rule for finding the date of Easter? Put simply, Easter is observed on the first Sunday after the first full moon on or after the day of the vernal equinox. The vernal equinox is the beginning of astronomical spring. However, ecclesiastical rules are slightly more complicated than this formula.