Patriot's Day was April 15th this year and commemorates the first battles of the American Revolution. It was originally April 19th, but more recently it has been celebrated on the 3rd Monday in April. It is not celebrated in all states, mostly in New England. The Boston Marathon is run on Patriot's Day.
Alas, I no longer live in New England and also they've rearranged the IRS service centers and the policies so this peculiar occurrence does not happen any longer, Patriot's Day or not.