Patriots' Day

Proper noun

 * 1)  A civic holiday commemorating the anniversary of the Battles of Lexington and Concord, the first battles of the American Revolutionary War on April 19, 1775. It was previously celebrated on April 19, but since 1969 has been observed on the third Monday in April.

Usage notes

 * Officially spelled Patriots' Day in Massachusetts and Wisconsin and Patriot's Day in Maine.