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A triduum (plural: tridua) is a religious observance lasting three days.
The best-known example today is the liturgical Paschal Triduum (the three days from the evening of Maundy Thursday to Easter Sunday). Other liturgical tridua celebrated in Western Christianity include the Rogation Days and the feasts of Christmas, Easter and Pentecost together with the first two days of their octaves, while in Eastern Christianity (both Orthodox and Catholic) the analogues of festive tridua take the form of a major feasts followed by an associated Synaxis. The most publicly celebrated examples are the feast of Epiphany together with its eve and the following day dedicated to Saint John the Baptist, and the Nativity feast with Christmas Eve and the Synaxis of Theotokos.
Many other tridua are celebrated on occasions such as when children are in preparation for their first Communion; among pupils at the beginning of the school year; among seminarians at the same time; and in religious communities for those who are to renew their vows yearly or every six months.