Per IRS Publication 17 Your Federal Income Tax (For Individuals), page 23:

Qualifying Widow(er)

If your spouse died in 2018, you can use married filing jointly as your filing status for 2018 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 in the instructions.

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 2017, and you haven't remarried, you may be able to use this filing status for 2018 and 2019.

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 2018 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 2016 or 2017 and you didn't remarry before the end of 2018.
• 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 2018:
  1. The child had gross income of $4,150 or more,
  2. The child filed a joint return, or
  3. 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, enter the child's name in the entry space at the far right of the filing status checkboxes (next to Qualifying widow(er)). 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 also are 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.