Midnight in your own time zone. After that, if your e-file is rejected, you have a few more days to fix it and re-file. But you cannot do a "first" e-file after midnight. An unfinished 2020 return will have to be finished using downloaded desktop software and then it has to be printed, signed and mailed if you miss the filing deadline.
Caution---do not wait til the last minute--something will almost inevitably go wrong and then you will be very upset.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
