Patriots' Day (otherwise known at Marathon Monday) commemorates the Battles of Lexington and Concord, the start of the Revolutionary War.
The holiday is observed this year on Monday, April 15. It's a state holiday, but not a federal holiday.
Here's a handy list of what's open - and closed - during the holiday:
Retail stores: Open at owner’s discretion.
Liquor stores: Open.
Convenience stores: Open.
Taverns, bars: Open.
Banks: Open at bank’s discretion; most open.
Stock market: Open.
Municipal and state offices: Closed.
Federal offices: Open.
Mail: Post offices open; regular delivery schedule.
Parking meters: Will not be in effect, but all other traffic rules apply.
Trash collection: one day delay.