09/01/2026
September 15 is two deadlines wearing one date.
Q3 estimated tax payment. If you are self-employed, or an S corp owner taking distributions, this is your third payment of the year.
Extended returns for partnerships (Form 1065) and S corps (Form 1120-S). If you filed an extension in the spring, this is the actual deadline. Not October. October is for individuals.
The second one catches S corp owners who assume they have until October because that is when their personal return is due. The business return is a month earlier, and it has to be finished before the personal one can be.
If you are not sure which of these applies to you, that is a fair question to ask now rather than on September 12.