Source : Webster's Revised Unabridged Dictionary (1913)

Labor Day \Labor Day\
   In most of the States and Territories of the United States, a
   day, usually the first Monday of September, set aside as a
   legal holiday, in honor of, or in the interest of, workingmen
   as a class. Also, a similar holiday in Canada, Australia,
   etc.

Source : WordNet®

Labor Day
     n : first Monday in September in U.S. and Canada