'Boxing Day' definitions:

Definition of 'Boxing Day'

(from WordNet)
noun
First weekday after Christmas

Definition of 'Boxing day'

From: GCIDE
  • 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.]