What does Boxing Day have to do with boxing?
Old English bryht, by metathesis from beorht "bright; splendid; clear-sounding; beautiful; divine," from Proto-Germanic *berhta- "bright" (cf. Old Saxon berht, Old Norse bjartr, Old High German beraht, Gothic bairhts "bright"), from PIE root *bhereg- "to gleam, white" (cf. Sanskrit bhrajate "shines, glitters," Lithuanian breksta "to dawn," Welsh berth "bright, beautiful"). Meaning "quick-witted" is from 1741.
Day; the daytime (1930s+ Black)