Tax Tips for Separated Couples - "December 31 is an important day for separated couples. The IRS considers you married for the entire tax year when you have no separation maintenance decree by the final day of the year. If you are married by IRS standards, you can only choose "married filing jointly" or "married filing separately" status. You cannot file as "single" or "head of household."" - https://turbotax.intuit.com/tax-tools/tax-tips/Family/Tax-Tips-for-Separated-Couples/INF22372.html
