Form 1120-S deadline: the S corporation filing calendar

Form 1120-S is generally due by the 15th day of the third month after the S corporation's tax year ends. A calendar-year S corporation would normally use March 15; in 2026 that date falls on a Sunday, so the filing deadline is March 16, 2026.

The S corporation also furnishes each shareholder a Schedule K-1, and Schedule K-3 when applicable, on the same general timetable. A delayed corporate return can therefore delay the owners' individual returns because the K-1 carries their share of income, deductions, and other tax items.

Form 7004 is generally used to request an automatic six-month extension for Form 1120-S. As with other business returns, an extension to file does not erase tax or payment obligations that may already be due.