This year, the rest of the country might finally learn about Emancipation Day – a little-known holiday beyond Washington that marks the anniversary of the abolition of slavery in the nation’s capital.

The holiday, which shuts down city government, will be celebrated April 15, also known as Tax Day. That means the entire country gets a three-day reprieve on filing its 2015 taxes, making the official tax filing deadline Monday, April 18.

Emancipation Day typically is celebrated April 16, the day in 1862 when President Abraham Lincoln signed the Compensated Emancipation Act, which freed about 3,100 slaves living in the District.

But this year, April 16 falls on a Saturday, so the District will celebrate the holiday on Friday, April 15. D.C. holidays are treated as federal holidays for tax-filing purposes, according to the Internal Revenue Service.

That pushes the official tax-filing deadline to the next business day, Monday, April 18. And, everyone – residents of Florida, California and everywhere in between – will receive an extension, not just D.C. residents.

“When April 15 falls on a Saturday, Sunday, or legal holiday, a return is considered timely filed if it is filed on the next succeeding day that is not a Saturday, Sunday, or legal holiday,” IRS spokesman Ubon Mendie in an email, referencing the U.S. law pertaining to the IRS. “The term ‘legal holiday’ includes a legal holiday observed in the District of Columbia.”

In 2007, the country had a two-day reprieve to file its taxes because April 15 fell on a Sunday and the next day – Monday, April 16 – was D.C. Emancipation Day.

Residents of Maine and Massachusetts will get an additional day on top of the three-day extension this year to file their taxes: Those states are celebrating Patriots’ Day – a holiday commemorating the 1775 battles of Lexington and Concord – on Monday, April 18.

Residents of those states will have until April 19 to file their taxes.

D.C. government shuts down for Emancipation Day, although the federal government and most private businesses in the city remain open. Councilmember Vincent Orange introduced legislation, and then-Mayor Anthony Williams declared Emancipation Day a D.C. holiday in 2000. The holiday was officially observed for the first time as a public holiday that closed the city government in 2005.

The holiday is most notably marked by a citywide parade in which city leaders and schoolchildren march along Pennsylvania Avenue.

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