Form 1120 is due on the 15th day of the 4th month after the end of the corporation’s tax year. Internal Revenue Service For calendar-year C-corporations, that means April 15, 2026. Corporations with a short tax year ending on June 30, 2026, must file by September 15, 2026, and may use Form 7004 to request an automatic 7-month extension of time to file. Internal Revenue Service