What does Boxing Day have to do with boxing?
early 15c., from Late Latin ternarius "consisting of three," from terni "three by three," from ter "thrice," which is related to tres "three" (see three).
ternary ter·na·ry (tûr'nə-rē)adj. Composed of three or arranged in threes, as a chemical compound containing three elements. n. A group of three.