What does Boxing Day have to do with boxing?
Old English glæm "brilliant light; brightness, splendor, radiance," from Proto-Germanic *glaimiz (cf. Old Saxon glimo "brightness;" Middle High German glim "spark," gleime "glowworm;" German glimmen "to glimmer, glow;" Old Norse glija "to shine, glitter"), from root *glim-, from PIE *ghel- "to shine, glitter, glow" (see glass).
early 13c., from gleam (n). Related: Gleamed; gleaming.