I believe there are some election filing date errors in @Mike9241 response or just date typos.
In order to be eligible for the Section 475 election for 2020, that election needed to be filed by the original due date for your 2019 form 1040. Typically that would have been April 15th of 2020. However, due to the extension of the original due date for 1040's, individuals had until July 15th of 2020 to make the Section 475(f) election for 2020 tax returns.
Since there is also an extension for the original due date for your 2020 form 1040, the Section 475(f) election must be made by May 17, 2021 in order to mark-to-market apply to 2021.
These dates only apply if you are an individual (or single member LLC treated as a disregarded entity). The election dates would be different if you were handling your day trading activity through an S corporation.
*A reminder that posts in a forum such as this do not constitute tax advice.
Also keep in mind the date of replies, as tax law changes.
