What does Boxing Day have to do with boxing?
"as, in the capacity of," from Latin qua "where? on which side? at which place? which way? in what direction?" figuratively "how? in what manner? by what method?; to what extent? in what degree?" correlative pronomial adverb of place, from PIE *kwo-, stem of relative and interrogative pronouns (cf. Old English hwa "who;" see who).