What does Boxing Day have to do with boxing?
early 15c., from Latin memorandum "(thing) to be remembered," neuter singular of memorandus "worthy of remembrance, noteworthy," gerundive of memorare "to call to mind," from memor "mindful of" (see memory). Originally a word written at the top of a note, by 1540s it came to stand for the note itself. The Latin plural is memoranda. Cf. also agenda.