The traditional tax-filing deadline is April 15 every year. That means that you have until that date to file a return for the calendar year that ended 3-1/2 months earlier. For example, April 15, 2009 is the tax-filing deadline for the year ended Dec. 31, 2008.
If you need fail to meet the deadline, you can obtain an extension by filing IRS Form 4868 by April 15. The extension extends the time to file, but not to pay. You should estimate your tax liability and send any amount you think you owe with the extension.
The extension grants you six additional months, to October 15, to file an individual income tax return. (If you are living outside of the U.S., you already have two additional months to file. In that case, submitting Form 4868 gives you an additional two-month extension.)
An extension grants you more time to file a return but it likely means paying a penalty on the amount of taxes owed but not paid at the time you file for an extension. For more information, see Form 4868.