The e-filed 2020 tax return has to be transmitted on or before midnight local time October 15, 2021.
The mailed 2020 tax return has to have a postmark of no later than October 15, 2021.
If the tax return has a tax refund or no taxes owed then there is no penalty for filing after the due date. The 2020 tax return can be filed at any time and if for a refund on or before May 17, 2024 or October 15, 2024 if an extension was filed.
Do not wait until the last minute to efile because there is almost always a backup and your return may not get thru on time. If you don't file at least 3 hours before midnight your time Murphy's Law will probably bite you in the backside.
It is better to efile then amend then to not efile at all.