What does Boxing Day have to do with boxing?
c.1300, "directions in religious services" (often in red writing), from Old French rubrique, rubriche "rubric, title" (13c.), from Latin rubrica "red ochre, red coloring matter," from ruber, from PIE root *rudhro- (see red). Meaning "title or heading of a book" is from early 15c. Related: Rubrical.