Deadline for corporate tax returns (Forms 1120, 1120A, and 1120S) for the year 2013, or to request automatic 6-month extension of time to file (Form 7004) for corporations who use the calendar year as their tax year. (The normal deadline for corporate returns is March 15th, but this falls on a Saturday, so the deadline is pushed to the next business day, which is Monday, March 17th.)
Final deadline to file an amended corporate tax return (Form 1120X) for tax year 2010 and still claim a tax refund. (Refunds expire three years from the original due date of the return.)