The marriage rule works like this: you are considered married for the entire year if you are married on December 31. Although lawmakers have made many changes to eliminate the differences between married and single taxpayers, in most cases the joint return will work to your advantage. Thus, wait until next year to finalize the divorce if alimony is not involved.
Warning on alimony! The Tax Cuts and Jobs Act (TCJA) changed the tax treatment of alimony payments under divorce and separate maintenance agreements executed after December 31, 2018:
Under the old rules, the payor deducts alimony payments and the recipient includes the payments in income.
Under the new rules, which apply to all agreements executed after December 31, 2018, the payor gets no tax deduction and the recipient does not recognize income. And if you are married on December 31, don’t file in April as married, filing separately. In most cases, this is a sure way to overpay your taxes.