The IRS always requires that taxes be filed on April 15th unless April 15th is a Saturday, Sunday, or federal holiday. This happens a lot more than you would think. In 2011, tax day was pushed to April 18th. April 15th, 2011 was a Friday and Emancipation Day was observed in Washington D.C. that Friday. Since the next two days were Saturday and Sunday, it was pushed to Monday the 18th. It was the 15th for 2008, 2009 and 2010 but was pushed to the 17th for 2006 and 2007.
So what about this year? Taxes are due on April 17th, 2012. April 15th is a Sunday this year and Emancipation Day will be observed the next day, the 16th, so taxes will need to be mailed by the 17th. Two extra days is better than no extra days!
Unfortunately, this also means that taxes will be due on April 15th for the next few years. 🙁