For Americans living within the US, the tax return is always due on April 15th or the following Monday (if the 15th is a weekend or a holiday). A two month extension is automatically given to citizens living abroad putting the expat due date (for filing purposes) at June 15th. The two month extension is automatic, but an additional extension can be filed for if needed. The filed-for extension moves the due date (for filing purposes) as far back as October 15th. Neither of these extensions apply to paying taxes. Any taxes owed, regardless of whether you are stateside or abroad, are due on April 15th.
There is a Monday, 6/17/13, deadline for U.S. citizens and resident aliens living overseas, or serving in the military outside the U.S. on the regular due date of their tax return. Eligible taxpayers get two additional days because the normal 6/15/13 due date falls on Saturday this year. To use the automatic two-month extension, taxpayers must attach a statement to their return explaining which of these situations applies.