The rest of the country might finally learn about D.C. Emancipation Day — a holiday little known beyond the District that marks the anniversary of the abolition of slavery in the nation’s capital.

This year, the holiday, which shuts down city government, will be celebrated April 15, also known as Tax Day. And because of the conjuction, the entire country gets a three-day reprieve on filing 2015 tax returns, making the official tax filing deadline Monday, April 18.

Emancipation Day usually is celebrated April 16, the date 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 April 15, a Friday. 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, 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 said in an email. “The term ‘legal holiday’ includes a legal holiday observed in the District of Columbia.”

In 2007, the country had a two-day reprieve on filing tax returns 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. Those states are celebrating Patriots’ Day — commemorating the 1775 battles of Lexington and Concord — on Monday, April 18. Residents of the two states will have until April 19 to file tax returns.

D.C. government shuts down for Emancipation Day, although the federal government and most businesses in the city remain open. In 2000, then-D.C. Council member Vincent B. Orange Sr. (D-Ward 5) prepared legislation —  which the council passed unanimously and then-Mayor Anthony A. Williams (D) signed — declaring April 16 D.C. Emancipation Day, a legal “private” holiday. The day was officially observed for the first time as a public holiday, one that closed the city government, in 2005.

The holiday is marked by a parade in which city leaders and schoolchildren march along Pennsylvania Avenue. This year, the rest of the country also might learn about the District holiday.