What does Boxing Day have to do with boxing?
1620s, "white," from Latin candidum "white; pure; sincere, honest, upright," from candere "to shine," from PIE root *kand- "to glow, to shine" (see candle). In English, metaphoric extension to "frank" first recorded 1670s (cf. French candide "open, frank, ingenuous, sincere"). Of photography, 1929. Related: Candidly; candidness.