Couples who are splitting up but not yet divorced before the end of the year have the option of filing a joint return. The alternative is to file as married filing separately. It's the year when your divorce decree becomes final that you lose the joint return option. In other words, your marital status as of December 31 of each year controls your filing status for that entire year.
If you can't file a joint return for the year because you’re divorced by year-end, you can file as a head of household (and get the benefit of a bigger standard deduction and gentler tax brackets), if you had a dependent living with you for more than half the year, and you paid for more than half of the upkeep for your home.