Marital Status is generally determined on the last day of the year.

A taxpayer is considered single for the entire tax year if, on the LAST DAY OF THE YEAR, he or she was:

UNMARRIED.

LEGALLY SEPARATED UNDER A DECREE OF DIVORCE OR SEPARATE MAINTENANCE.

LEGALLY SEPARATED BY THE END OF THE YEAR.

EXAMPLE

Jose and Maria lived together until June 30, when they separated. Jose filed for the divorce on July 31, and the divorce became final on December 28, 2022.

They don't have any dependents. They each must file as "single" for 2022.

The fact that the divorce became final before the end of the year, so they unmarried for tax purpses in 2022.