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 the Eastern Christianity (both Orthodox and Catholic) have an equal to a festive triduum when a major feast is followed by a Synaxis. The most publicly celebrated ones are the one of the feast of Epiphany together with its eve and the following day dedicated to Saint John the Baptist, and the Nativity feast with the Christmas Eve and the Synaxis of Theotokos.
Ecclesiastical approval has been granted for tridua in honour of the Trinity, the Eucharist and Saint Joseph.
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.