What does Boxing Day have to do with boxing?
early 15c., "pertaining to the walls of a cavity in the body," from Late Latin parietalis "of walls," from Latin paries (genitive parietis) "wall" (of a building), of unknown origin. In U.S. also "pertaining to the residents and rules of a college or university" (1837).
parietal pa·ri·e·tal (pə-rī'ĭ-tl)
Relating to or forming the wall of any cavity.
Of or relating to either of the parietal bones.