The following are tax deadlines due in September and October:
September 16th is the deadline for the third quarter installment for individuals & corporations paying estimated taxes throughout the year.
September 16th is the deadline for Forms 1120S (S Corporations) & 1065 (Partnerships) for S Corporations and Partnerships that requested a 6-month extension on their returns. This is the deadline to file that return and provide each partner with a Schedule K-1.
October 15th is the deadline for individuals & corporations that requested a 6-month extension to file their 2018 tax returns.
October 15th is the deadline to make SEP IRA contributions if you filed for a request for an extension to file your return.
If you owe taxes and miss extension tax deadlines you will be charged failure to file and failure to pay penalties which are added to your account until you file your tax return.
If you miss a deadline and are due a refund, there is no penalty. The government will hold your money interest free. If you file 3 years past your filing deadline you will loose your refund (this is a statute of limitations rule).