In the United States, Tax Day is a colloquial term for the day on which individual income tax returns are due to the federal government. The term may also refer to the same day for states, even where the tax return due date is a different day.

Since 1955, for those living in the United States, Tax Day has typically fallen on April 15. For those filing a U.S. tax return but living outside the United States and Puerto Rico, Tax Day has typically fallen on June 15, due to the two-month automatic extension granted to filers by IRS Publication 54.

From Wikipedia