a legal holiday in the U.S. commemorating the discovery of America by Columbus (Oct. 12, 1492), observed in most states on the second Monday in October
See Columbus Day in American Heritage Dictionary 4
noun
October 12, observed in the United States in commemoration of the landing by Christopher Columbus in 1492 in America, which became known as “the New World” because Europeans did not know that it had existed. Columbus Day is now officially observed on the second Monday in October.