If your spouse passes during the year; both are considered married for the entire year and for filing status purposes.
Now if the surviving spouse remarries before the end of the year, the dead spouse if forced to have a tax return filed on their behalf as “Married Filing Separately”.
I am actually aware of a few instances where individuals remarried very quickly; but not yet in the same year. But the statistics show that those who were in long happy relationships do remarry.