Your marital status for the whole year is what you were as of midnight Dec 31, 2020.   It does not matter if you were married on Dec, 31 you are still married for all of 2020.

If married, then your only filing choices are Married filing jointly or Married Filing Separately.     Separately has many limitations and the overall tax will probably be higher.

if you are married at the end of 2020, (or if you are widowed in 2020) you can file a joint return or each of you can use the married filing separate status.   filing joint usually produces the lowest combined taxes.  on the return you include your income for the whole year even the period before marriage.