Form 1120S generally must be filed by March 15 of the year immediately following the calendar year covered by the return or, if a fiscal year (a year ending on the last day of a month other than December) is used, by the 15th day of the third month immediately following the last day of the fiscal year. The corporation must complete a Schedule K-1 for each person who was a shareholder at any time during the tax year and file it with the IRS along with Form 1120S. The second copy of the Schedule K-1 must be mailed to the shareholder.