Complex sentence: In early traditions, students elected one of their classmates as "bishop" on St. Nicholas Day, who would rule until 28 December (Innocents Day), and they sometimes acted out events from the bishop's life.
Split into simpler sentences: In early traditions, students elected one of them as "bishop" on St. Nicholas Day, who would rule until 28 December (Innocents Day). They sometimes acted out events from the bishop's life.