The Collaborative International Dictionary of English v.0.48:

Dominion Day \Do*min"ion Day\
   In Canada, a legal holiday, July lst, being the anniversary
   of the proclamation of the formation of the Dominion in 1867.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Dominion Day
    n 1: a legal holiday in Canada commemorating receiving Dominion
         status in 1867 [syn: Dominion Day, July 1]