Calendar year-end returns or extensions are due by the 15th day of the 4th month after end of tax year (4/15). Form 4868 grants an automatic 6-month extension (10/15).
Note: If a deadline falls on a weekend or government holiday, the deadline will be pushed to the next business day per IRS instructions.
This line from the end of the topic linked to a page that can no longer be found on the IRS website:
Refer to the IRS Calendar for more information. (link was to a page called Tax-Year-2011-e-file-Calendar)