Patriots' Day is observed on April 19, 2021. Patriots' Day (officially Patriots' Day in Massachusetts and Patriot's Day in Maine) is a civic holiday commemorating the anniversary of the Battles of Lexington and Concord, the first battles of the American Revolutionary War. It is observed on the third Monday in April in Massachusetts and Maine, and is a public school observance day in Wisconsin.
Since 1969, the holiday has been observed on the third Monday in April, providing a three-day long weekend, as well as being the first day of public school vacation week in Maine and Massachusetts. Previously, it had been designated as April 19, the actual anniversary of the battles.
Patriots' Day is also a school holiday for many local colleges and universities, both public and private. In Maine and Massachusetts, when Patriots' Day falls on a day where income tax returns would otherwise be due for the remainder of the country, residents of those states are given until midnight of the next day (Tuesday) to submit their state tax returns. (With material from: Wikipedia)