Pope Innocent III (1216, Homilia; PL, vol. CCXVII, col. 393)
By custom, priests wear rose vestments and flowers may be returned to the altar on this day which takes its joyful cue from the entrance antiphon for the Mass: Laetare Jerusalem (Rejoice, O Jerusalem).
During the Middle Ages it became customary to treat Laetare Sunday as a Mother's Day -- celebrating the Mother Church as well as one's own mother with flowers and simnel cake.
This is supposed to be a plum cake, but you'd never know it from the recipe which is so labor intensive that I'm going to opt to drink a glass of prune juice instead. While wearing pink! of course.