The Collaborative International Dictionary of English v.0.48:

Boxing day \Box"ing day`\
   The first week day after Christmas, a legal holiday on which
   Christmas boxes are given to postmen, errand boys, employees,
   etc. The night of this day is boxing night. [Eng.]
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Boxing Day
    n 1: first weekday after Christmas