IRS Publication 17, Your Federal Income Tax For Individuals, page 21:
You are considered married for the whole year if, on the last day of your tax year, you and your spouse meet any one of the following tests.
- You are married and living together.
- You are living together in a common law marriage recognized in the state where you now live or in the state where the common law marriage began.
- You are married and living apart, but not legally separated under a decree of divorce or separate maintenance.
- You are separated under an interlocutory (not final) decree of divorce.