Visitors to the Newseum on Pennsylvania Avenue got a view of the Emancipation Day parade on April 16. The holiday falls on a Friday next year. (Michael S. Williamson/The Washington Post)

Next year, the whole country might finally learn about Emancipation Day — a D.C. holiday that is little known beyond the city.

The holiday, which marks the anniversary of the abolition of slavery in the District, will officially be celebrated April 15, 2016, giving the entire country a three-day reprieve on filing its 2015 taxes.

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

But next year, April 16 falls on a Saturday, which means the District will officially be celebrating the holiday on Friday, April 15 — better known as Tax Day. And according to the Internal Revenue Service, D.C. holidays are treated as federal holidays for tax-filing purposes.

That pushes the official tax-filing deadline to the next business day, Monday, April 18. That’s right: Everyone — residents of Florida, California and everywhere in between — will receive an extension.

“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 wrote in an e-mail, 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 similarly 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.

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

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

Former D.C. mayor Anthony A. Williams declared Emancipation Day an official District holiday 15 years ago.

The city government shuts down for Emancipation Day, although the federal government and most private businesses in the city remain open, so the holiday isn’t a high-profile one.

The holiday is most notably marked by a citywide parade in which city leaders and schoolchildren march along Pennsylvania Avenue celebrating the city. This year, there was also a free concert and fireworks.

“Everyone, no matter their background, celebrates Emancipation Day,” Muriel E. Bowser (D), now the District’s mayor, said last month on Emancipation Day.

Next year, that will be true across the country.