Memorial Day is the only U.S. federal holiday in May and is celebrated on the last Monday of the month. Some days that people consider holidays are actually days of observance. Mother's Day, Cinco de Mayo, and the National Day of Prayer are observance days that take place in May.
Memorial Day was originally called Decoration Day and observed on the May 30, when flowers were laid on the graves of Civil War veterans. The holiday now commemorates the sacrifice of all those who have died while protecting the United States.
The Jewish holidays Yom Ha'atzmaut and Lag BaOmer are sometimes celebrated in May, depending on the dates of the Jewish calendar. Primary Election Day is a state holiday in Indiana, as is Truman Day in Missouri.