What does Boxing Day have to do with boxing?
1863, "devised beforehand," past participle adjective from calculate. Earlier, "suited, apt" (1722).
1560s, "to compute, to estimate by mathematical means," from Latin calculatus, past participle of calculare "to reckon, compute," from calculus (see calculus). Meaning "to plan, devise" is from 1650s. Replaced earlier calculen (mid-14c.), from Old French calculer. Related: Calculable.