The filing deadline for a 2019 return--the deadline by which you must PAY your estimated 2019 tax -- is July 15, 2020. If you get an extension to file, it does not give you more time to pay, but you will be able to e-file up to October 15, 2020. After that, e-filing will shut down and 2019 returns will have to be filed by mail.
since 7/15/2020 has not yet arrived that is the earliest a federal return is due. That's also the date any balance due must be paid to avoid late payment penalties and interest. filing an extension gives you til 10/15/2020 only to file. states have different dates.