TTX online shuts down for the 2016 tax filing season on Oct 16 at midnight pacific time. So don't bother with it. Since you have to use the CD/Desktop version to file a prior year's return anyway, you might as well go ahead and purchase the desktop version for 2016 too. I assume you already know to file the oldest tax return first, and that you should mail each tax return in a physically separate envelope to the IRS.
These are the latest dates and times TurboTax posted for SuperUsers to use concerning end of filing season:
October 16, 2017 at 11:59:59 PM local time to the customer- The final deadline to E-file your return for this tax year and be considered “on time” (assuming you filed an extension).
October 17, 2017 at 10 AM PST– New Users and Not Started (not complete?) will see a desktop pre-order focused homepage, with a message about TTO being available in Dec. and an opportunity to create a TTO account.
October 22, 2017 at 3 AM PST- Last day to fix rejected returns and not incur additional penalties or interest (again assuming an extension was filed and accepted)
October 26, 2017 at 3 AM PST– E-file servers are shut down for the year for personal returns. No exceptions!
October 26, 2017 at 3 PM PST- Customers will no longer be able to sign into TurboTax Online itself, but still can access their MyTurboTax.
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NOTE: Remember all tax data files and PDF tax documents are very sensitive files, since they contain your personal ID info, financial data, and possibly bank account numbers, etc. Be sure to store them safely and securely to guard against computer theft, hacking, etc.