Per IRS Publication 17 Your Federal Income Tax (For Individuals), Chapter 3, on page 25:

Qualifying Widow(er)

If your spouse died in 2020, you can use married filing jointly as your filing status for 2020 if you otherwise qualify to use that status. The year of death is the last year for which you can file jointly with your deceased spouse. See Married Filing Jointly, earlier.

You may be eligible to use qualifying widow(er) as your filing status for 2 years following the year your spouse died. For example, if your spouse died in 2019, and you haven't remarried, you may be able to use this filing status for 2020 and 2021.

This filing status entitles you to use joint return tax rates and the highest standard deduction amount (if you don’t itemize deductions). It doesn't entitle you to file a joint return.

Eligibility rules. You are eligible to file your 2020 return as a qualifying widow(er) if you meet all of the following tests.

• You were entitled to file a joint return with your spouse for the year your spouse died. It doesn't matter whether you actually filed a joint return.
• Your spouse died in 2018 or 2019 and you didn't remarry before the end of 2020.
• You have a child or stepchild (not a foster child) whom you can claim as a dependent or could claim as a dependent except that, for 2020:
  a. The child had gross income of $4,200 or more,
  b. The child filed a joint return, or
  c. You could be claimed as a dependent on someone else’s return.
• If the child isn't claimed as your dependent in the Dependents section on Form 1040 or 1040-SR, enter the child's name in the entry space at the bottom of the Filing Status section. If you don’t enter the name, it will take us longer to process your return.
• This child lived in your home all year, except for temporary absences. See Temporary absences, earlier, under Head of Household. There are also exceptions, described later, for a child who was born or died during the year and for a kidnapped child.
• You paid more than half of the cost of keeping up a home for the year. See Keeping Up a Home, earlier, under Head of Household.

After the two years, you may be able to file as Head of Household, if you qualify.

Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.