If you are a Partnership or S-Corp, your due date for tax filing is 3/15. If you are under Schedule Cs or C-Corp, your date is due by the 15th of April, which if falls on a holiday, is shifted to following business day.

The personal tax or 1040 becomes due every April 15th, which if falls on a holiday, is shifted to following business day.

For business taxes, if you file an extension on 3/15 or 4/15, and if you are S-Corps or a Partnership, it extends till 9/15 for and till 10/15 if you are C-Corps or under personal returns.