The very good reason taxes are due on April 18, not April 15, this year

If you haven't touched your taxes yet this year, here's some good news: A holiday celebrated only in Washington, DC, has bought you a few extra days. For most Americans, federal taxes are due Monday, April 18, rather than Friday, April 15.

That's because tax filing deadlines are pushed back to the next business day if April 15 falls on a weekend, a federal holiday, or a holiday celebrated in Washington, DC.

And this year April 15 is a holiday in the District of Columbia. Emancipation Day, usually celebrated on April 16, falls on a Saturday, so it's being observed on Friday, April 15 — Tax Day — instead. (Maine and Massachusetts residents have until April 19 to file their taxes: April 18 is Patriot's Day, which commemorates the battles of Lexington and Concord during the Revolutionary War.)

But if you only know about Emancipation Day because it gave you a break on rushing to finish your taxes, you're missing that the holiday itself has a fascinating history.

Emancipation Day celebrates the end of slavery in Washington, DC

In 1862, eight months before the Emancipation Proclamation, Congress passed a law ending slavery in Washington, DC — a long-held goal of abolitionists, including President Abraham Lincoln, who had been fighting to end slavery in the nation's capital since 1849.

Lincoln signed the law on April 16, 1862. At the time, about 3,300 slaves were living in the District, outnumbered two to one by free black residents. To free them, Congress paid slaveholders an average of $300 per slave. And to get their money, slaveowners had to file petitions with details about the slaves that were going to be freed.

Those petitions have been preserved. The result is a detailed record, a reminder of the everyday horrors of slavery and of the wealth that slavery perpetuated: As the Civil War began, slaves were more valuable than all of the agricultural land in the US.

One slaveowner listed her slaves this way:

William Johnson, Mulatto about 5 ft 2 in high, 15 years old & sound

Joseph Johnson Mulatto 4 ft 3 in high 10 years old & sound

Sallie Butler Dark Mulatto 5 ft high 19 years old

Martha Wharton Mulatto 5 ft 4 in high 38 years old

Phillis, Black 4 ft 6 in high 35 years old

Lewis, Black 5 ft 10 in high 24 years old…

That your petitioner's claim to the service or labor of said persons was, at the time of said discharge therefrom, of the value of 5400 dollars in money.

William Johnson is a good ploughboy & handy with horses

Joseph Johnson is a good house boy

Martha is a good cook, washer & ironer and seamstress

Lewis is a first rate coach painter and performer on the violin being the leader of a band.

Phillis is a first rate cook washer & ironer

nor do I know of any bodily infirmity or defect of either of them as to impair their value.

That $5,400 in 1862 is worth nearly $150,000 in 2016. The University of Nebraska has digitized the petitions and put them all online; they're well worth reading.

Why DC's Emancipation Day is special

Emancipation Day only celebrates the end of slavery in Washington, DC — for the rest of the country, the traditional day to commemorate it is Juneteenth, June 19, which celebrates the end of slavery in Texas.

But although at least 42 states have some sort of way to mark Juneteenth, nobody celebrates it the way they do major holidays like Memorial Day: Even in Texas, where state workers can take it as an official holiday, some people are required to work to maintain state business.

DC, on the other hand, goes all in for Emancipation Day, closing schools and offices (although the DC Metro runs on a normal schedule). And this is important for more reasons than the break it gives people on filing their taxes. Slate's Jamelle Bouie explained why the end of slavery is worth celebrating:

Insofar that modern Americans celebrate the past, it’s to honor the sacrifices of the Greatest Generation or to celebrate the vision of the Founders. Both periods are worthy of the attention. But I think we owe more to emancipation and the Civil War. If we inaugurated freedom with our nation’s founding and defended it with World War II, we actualized it with the Civil War. Indeed, our struggle against slave power marks the real beginning of our commitment to liberty and equality, in word, if not always in deed.

Put another way, Juneteenth isn’t just a celebration of emancipation, it’s a celebration of that commitment. And, far more than our Independence Day, it belongs to all Americans.