Christ redeemed humankind and gave perfect glory to God principally through his Paschal Mystery: dying he destroyed our death and rising he restored our life. Therefore the Easter triduum of the passion and resurrection of the Lord is the culmination of the entire liturgical year. Thus the solemnity of Easter has the same kind of preeminence in the liturgical year that Sunday has in the week.
The Easter triduum of the passion and resurrection of the Lord begins with the Evening Mass of the Lord’s Supper, reaches its high point in the Easter Vigil, and closes with Evening Prayer on Easter Sunday, the Sunday of the Lord’s resurrection. On Good Friday and, if possible, also on Holy Saturday until the Easter Vigil, the Easter fast is observed everywhere.
The Easter Vigil, during the holy night when the Lord rose from the dead, ranks as the “mother of all holy vigils.” Keeping watch, the Church awaits Christ’s resurrection and celebrates it in the sacraments. Accordingly, the entire celebration of this vigil should take place at night, that is, it should either begin after nightfall or end before the dawn of Sunday.
Sacred Congregation of Divine Worship, General Norms for the Liturgical Year and the Calendar, n18-21
The Latin word 'triduum' refers to the 'three great days' in the Church’s calendar, Holy Thursday, Good Friday and Easter Sunday. These three days recall the events of the night before Christ’s death, his passion and death, and his resurrection.
On Holy Thursday the last supper Jesus had with the apostles is commemorated, during which he instituted the Eucharist and the priesthood.
Good Friday commemorates the events of the passion and death of Jesus, beginning in the Garden of Gethsemane and ending with his crucifixion and death on Calvary.
Holy Saturday is a time of prayer and fasting, and meditation focused on the death and burial of Jesus. Masses are not usually celebrated on this day, and in churches the altar is stripped and the tabernacle open.
The celebration of Easter Sunday begins with the Easter Vigil, held after darkness falls on the night of Holy Saturday. The Easter fire is blessed by the celebrant and the paschal candle is lit from it and carried into the darkened church, symbolising the light of Christ which dispels the darkness of sin and death. The singing of the Exultet is followed by readings, psalms and prayers. When these are finished the church is lit and the Mass of the Resurrection begins.
After the Gospel and homily the baptismal water is blessed and catechumens are baptised and confirmed. Candidates for initiation into the Church are confirmed, and the congregation renews their baptismal vows. The Mass is joyous and the Alleluia is heard for the first time since the beginning of Lent.
Easter Sunday, and therefore the Triduum, varies in date from year to year. The Council of Nicea in 325AD decreed that the day of Easter is to be celebrated on the Sunday that follows the first full moon after the vernal equinox, the day in Spring when there is a 12 hour day and a 12 hour night (March 20). This means that Easter can be as early as 22 March and as late as 25 April.
The liturgical colour of Easter is white.