Divorced in December - Do I file using joint or single filing status ?

If you are divorced during the tax year either by a final decree of divorce or separate maintenance agreement, you are treated as unmarried for the whole year.
If you remarry before the end of the year you will be treated as filing a joint return or you may elect to file as married filing a separate return.
An unmarried person with a child may use the filing status of "head of household." An unmarried person without a child will be treated as single. Speak to your local CPA before you file your tax return.