Once again, the deadline to file your US Federal income taxes is later than you might think. Last year, federal income tax returns were due on the 18th of April, and this year they are due on Tuesday, 17 April 2012. Why the 17th? Well, the 15th is on a Sunday, which would usually push the deadline to Monday. However, the 16th is a holiday for the District of Columbia, who celebrates Emancipation Day on the 16th of April each year. Under federal law, DC holidays are treated like federal holidays for purposes of income tax deadlines.
If you file an extension, your return will still be due on 15 October 2012 with payments being due on 17 April 2012. The change to the original due date does not translate into a corresponding change for an extension's expiration, and you always have to make the payment when it is originally due if you do not want to pay penalties and interest.
Don't forget that there are a few was in which military members can file later without filing for an extension. All taxpayers who live overseas automatically get two extra months to file.
If you or your spouse are serving in a combat zone, there is an automatic extension for you as well. (You have to be filing jointly for the spouse to benefit from this extension). The extension moves the due date to 180 days after the day in which the qualifying service member leaves the combat zone, plus an additional extension if they entered the combat zone between 1 January and the tax due date for that year. That additional extension is calculated by counting the number of days between the day you entered the combat zone and the due date for the taxes. Take the day that you entered the combat zone, add 180 days, then add the extra amount calculated separately. That's when your taxes are due. If you need more information on how this extension works, check out Publication 3, the Armed Forces Tax Guide.
If you decide that you need to file an extension, there are a few ways you can do it. The easiest way is to use the Free File link at IRS.gov to submit a Form 4868, the Application for Automatic Extension of Time to File U.S. Individual Income Tax Return. If you don't like the online option, you can print out a Form 4868 and mail it in, just make sure it is postmarked by Tuesday and you include your payment if you estimate that you will owe. Better to overpay when you estimate than to underpay when you estimate.
As a super-duper procrastinator, I encourage you to file before the deadline if you are able. If you really have to extend, make sure you do it by Tuesday.