Most years, taxes are due on April 15. If that day is a weekend or a federal holiday, then the next business day is when we all have to pony up to the IRS.
But in 2011, taxes are due on April 18. Why, you may ask? Because of Emancipation Day. Normally celebrated on April 16, this holiday marks the anniversary of the signing of the Compensated Emancipation Act by President Abraham Lincoln in 1862. This declaration freed the roughly 3100 slaves in the District of Columbia. Since it falls on a Saturday in 2011, it is being celebrated April 15, pushing the tax filing day back to Monday, April 18.
Interestingly, Emancipation Day is celebrated on different dates in different areas. Texas celebrates it on June 19, Mississippi on May 8 and Florida on May 20. All mark the dates when slaves first learned of their freedom, many by the first reading of the Emancipation Proclamation in 1865.
Emancipation Day was declared an official public holiday in the District of Columbia in 2005 and will continue to futz with the tax filing deadline every couple of years, since D.C. is where the IRS calls home. In 2007, Emancipation Day pushed back the deadline from April 16 to April 17, as April 15 fell on a Sunday. That same thing will happen again in 2012, so next year you get two extra days. This year, you get three.