In the past, IRS has shut down its efile service in late November to mid December and usually reopens prior year efiling with the opening of the next year's efiling season ... usually in mid to late January. (2020 was unusual, as the prior years' efile service didn't open up until April or May) After that reopening date, the prior-year efile system stays open for quite a while. In the past, I have efiled up to 3 years after the due date. In late fall, 2020, I successfully efiled a 2017 and 2018 Form 1040. In the summer of 2020, I successfully efiled several 2016 Form 1040 returns. It was accomplished by merely using the normal ProSeries efile routine. The problem is remembering to go back to the prior year's program an verifying the return as accepted ... and timely resubmitting if required.