Income taxes are due April 15. Except when they’re not. This year’s tax date, April 17, is the result of a convergence of a weekend and a little known legal holiday in Washington, D.C.

In 2018, April 15 falls on Sunday. In most years when Tax Day falls on a weekend, it moves to Monday. This year, however, Monday, April 16, is an important public holiday in the nation’s capital called Emancipation Day. So taxes will be due Tuesday, April 17.

Emancipation Day

President Abraham Lincoln signed the Compensated Emancipation Act on April 16, 1862, freeing nearly 3,000 slaves in the District of Columbia. Although formal slavery did not officially end in the U.S. until after the end of the Civil War in 1865, Lincoln’s action in 1862 is celebrated annually on April 16 in Washington D.C. 

Last year, when Emancipation Day fell on Sunday, observance of the holiday was moved to Monday, the closest workday and tax day became Tuesday, April 18. And next year, in 2019, tax day will return to April 15 for the first time since 2015.

Why April 15?

Tax Day hasn’t always been April 15. In 1913 the 16th Amendment to the Constitution created the modern tax system. At that time Tax Day was set as March 1. In 1918, 5 years later, the date was changed to March 15.

In 1955 Tax Day became April 15, supposedly to “spread the workload" for IRS employees. Many tax experts believe the real reason was to allow the government to keep your money longer before issuing a refund. 

A Whole Lot of Shifting Going On

When April 15 falls on a Saturday, Sunday or a civil holiday, the deadline for filing taxes is moved forward to the next workday. Patriots’ Day, celebrated in Massachusetts and Maine on the third Monday in April, sometimes conflicts with Tax Day, in which case residents of those states can file a day later.

Since 1955 there have been 63 Tax Days (including this year). Of those, 17 have been on days other than April 15. This means roughly 1 out of every 4 Tax Days does not fall on April 15.

And if you want to start filing early, the IRS announced that the 2018 tax filing season begins on January 29, 2018.