Electronically filed tax returns are accepted until October 15th.
If April 15th falls on a weekend or federal holiday, you have until midnight the following business day to submit your extension request or your tax return.
However, for your return to be considered timely after April 15, you must file Form 4868
(PDF), Application for Automatic Extension of Time to File, on or before April 15th.
With a timely filed extension request, you have until October 15th to timely file your return.
If an extension of time to file has not been timely submitted you may be subject to a failure to file penalty if you file your return after April 15th.