In the Eastern Church, Catholic and Orthodox, the day after a major feast day is itself a special one. Known as the “leave-taking” of the feast, this day celebrates one or more of the saints who were involved in the event that the feast celebrates.
Thus March 26, the Leave-Taking of the Annunciation, is dedicated to the Archangel Gabriel, the messenger who announced to the Blessed Virgin Mary that God had chosen her to bear his son.