According to the IRS in their first News Release of 2011:
IR-2011-1, Jan. 4, 2011
WASHINGTON ? The Internal Revenue Service today opened the 2011 tax filing season by announcing that taxpayers have until April 18 to file their tax returns.
Taxpayers will have until Monday, April 18 to file their 2010 tax returns and pay any tax due because Emancipation Day, a holiday observed in the District of Columbia, falls this year on Friday, April 15. By law, District of Columbia holidays impact tax deadlines in the same way that federal holidays do; therefore, all taxpayers will have three extra days to file this year. Taxpayers requesting an extension will have until Oct. 17 to file their 2010 tax returns.
The IRS expects to receive more than 140 million individual tax returns this year, with most of those being filed by the April 18 deadline.