What does Boxing Day have to do with boxing?
Old English Sæterdæg, Sæternesdæg, literally "day of the planet Saturn," from Sæternes (genitive of Sætern; see Saturn) + Old English dæg (see day). Partial loan-translation of Latin Saturni dies "Saturn's day" (cf. Dutch Zaterdag, Old Frisian Saterdi, Middle Low German Satersdach; Irish dia Sathuirn, Welsh dydd Sadwrn). The Latin word itself is a loan-translation of Greek kronou hemera, literally "the day of Cronus."
Unlike other day names, no god substitution seems to have been attempted, perhaps because the northern European pantheon lacks a clear corresponding figure to Roman Saturn. A homely ancient Nordic custom, however, seems to be preserved in Old Norse laugardagr, Danish lørdag, Swedish lördag "Saturday," literally "bath day" (cf. Old Norse laug "bath").
German Samstag (Old High German sambaztag) appears to be from a Greek *sambaton, a nasalized colloquial form of sabbaton "sabbath," also attested in Old Church Slavonic sabota, Polish sobota, Russian subbota, Hungarian szombat, French samedi.
Saturday night has been used figuratively to suggest "drunkenness and looseness in relations between the young men and young women" since at least mid-19c. Saturday-night special "cheap, low-caliber handgun" is American English, attested from 1976 (earlier Saturday-night pistol, 1929).