What does Boxing Day have to do with boxing?
1670s, shortening of registrary (1540s), from Medieval Latin registrarius "one who keeps a record" (related to register (n.)). Earlier were registerer (mid-15c.), registrer (late 14c.).
registrar reg·is·trar (rěj'ĭ-strär', rěj'ĭ-strär')n. An admitting officer in a hospital.