The Collaborative International Dictionary of English v.0.48:

Columbus Day \Co*lum"bus Day\
   The 12th day of October, on which day in 1492 Christopher
   Columbus discovered America, landing on one of the Bahama
   Islands (probably the one now commonly called Watling
   Island), and naming it "San Salvador"; -- called also
   Discovery Day. This day is made a legal holiday in many
   States of The United States.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Columbus Day
    n 1: a legal holiday commemorating the discovery of America by
         Christopher Columbus [syn: Columbus Day, Discovery Day,
         October 12]