If you are legally married as of 12/31/2020 you will file your 2020 tax return as either Married Filing Jointly or Married Filing Separately. You cannot file your 2020 tax return as Single if you are legally married.
On your 2020 tax return, that is filed in 2021, you enter the name that is on your Social Security card, regardless if it is your maiden name or married name.
On the W-4 you can go ahead and change your name before receiving the Social Security card with your married name.
The name that is on your W-2 from your employer does not have to match the name on your tax return. Only the Social Security numbers need to be the same on both forms.
