The calendar of saints is a traditional Christian method of organizing a liturgical year by associating each day with one or more saints and referring to the feast day of said saint. The system arose from the early Christian custom of annual commemoration of martyrs on the dates of their deaths, or birth into heaven, and is thus referred to in Latin as dies natalis ("day of birth"). In the Eastern Orthodox Church, it is called a Menologion. Menologion may also refer to a set of icons on which saints are depicted along the order of dates of their feasts, often made in two panels.

As the number of recognized saints increased during Late Antiquity and the first half of the Middle Ages, eventually every day of the year had at least one saint who was commemorated on that date. To deal with this increase, some saints were moved to alternate days in some traditions or completely removed, with the result that some saints have different feast days in different calendars. For example, Ss. Perpetua and Felicity died on 7 March, but this date was later assigned to St. Thomas Aquinas, allowing them only a commemoration (see Tridentine Calendar), so in 1908 they were moved one day earlier. When the 1969 reform of the Catholic calendar moved him to 28 January, they were moved back to 7 March (see General Roman Calendar). Both days can thus be said to be their feast day, in different traditions. The Roman Catholic calendars of saints in their various forms, which list those saints celebrated in the entire church, contains only a selection of the saints for each of its days. A fuller list is found in the Roman Martyrology, and some of the saints there may be celebrated locally.