The due date for filing 2011 United States tax returns is 17 April 2012 (without extension of time).
The usual date,15 April falls on a Sunday, and the 16th is the Emancipation Day holiday in DC, so the date is 17 April.
An automatic 2-month extension until June 15, 2012, applies to United States citizens and resident aliens residing outside of the United States on 17 April, the regular due date of the return. Note – the Internal Revenue Service will still charge interest on any amount of tax unpaid by 17 April 2012.
An extension to 15 October 2012 may be obtained by either making a credit card payment by 17 April of your estimated tax due, or by filing Form 4868 by 17 April 2012. Interest will also still apply on amounts of tax unpaid by 17 April 2012.