The initial due date for filing your US tax return for all US citizens, greencard holder and other residents is April 15, regardless of where you reside.
There is a common misconception that the due date is June 15 for expats. This is partially true – if you are living outside of the US on April 15, you have an “automatic extension” to June 15 for filing your US tax return. This automatic extension means you can temporarily avoid both the late filing and the late payment penalties (but not interest) even if you do not file an extension. However, if you take advantage of the automatic extension, it is important that you attach a statement to your tax return explaining that you have done so. Note that the automatic extension may not be valid for your state tax return, if applicable.
If you need additional time (i.e. you cannot file by June 15), you need to file Form 4868 by June 15 (www.irs.gov/pub/irs-pdf/f4868.pdf). There is a box to tick on this form when you were living outside of the US on April 15. This form gives you until October 15 to file your tax return.
As an expatriate, you can even get an additional extension until December 15. There is no special form for this (in the past you used the now-obsolete Form 2688). In order to obtain the additional two month extension you need to write a letter to the IRS explaining your situation and why you need the additional two months. If they approve your letter, you won’t hear anything. It is a good idea to send this letter by registered mail or courier so that you can show proof of mailing if necessary.
Finally, in case this wasn’t all confusing enough, there is a special extension possible for expatriates who moved out of the US during the tax year and require additional time to qualify for the “Foreign Earned Income Exclusion” using either the “Bona Fide Residence Test” or the “Physical Presence Test”. You apply for this extension using Form 2350 (www.irs.gov/pub/irs-pdf/f2350.pdf). With this form, you are able to request an extension until 30 days after you qualify for the Foreign Earned Income Exclusion. If you are using the Bona Fide Residence Test, this means you have until January 30 of the following year (e.g. for 2007 tax return, you get an extension to January 30, 2009).
Additional information can be found in Publication 54, Tax Guide for US Citizens and Resident Aliens Abroad (www.irs.gov/pub/irs-pdf/p54.pdf).