For income tax return purposes, your filing status is determined by the last day of the tax year--not by the tax filing deadline. So if you want to file as married for a 2020 tax return you have to be legally married by December 31, 2020. If you wait and get married in 2021 then for 2020's tax returns you are both Single, and for 2021 (which you file in 2022) you can file as married--assuming you get married in 2021.
Best Wishes!
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