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 day as that saint's feast day. The system arose from the very 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").
Calendar of Saints Feast Days used in the Dominican Convent at Tukku, Finland, about 1340-1360
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. The General Roman Calendar, in its various forms, contains only a selection of the saints for each of its days.