Different tax forms are due at different times throughout the year. For business filings, the major dates to know are:
February 1 - Form W-2 and Form 1099-NEC are due
March 15 - Form 1120-S and Form 1065 are due. This is also the deadline for filing an extension for either of these forms
April 15 - Form 1120 is due, and on most years, so are employees' tax returns
October 15 - extension deadline for C corps