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Generally, your marraige status is looked at as of the last day of the year. If as of that date you are not legally separated (i.e. divorced) or subject to a separate maintenance decree, then you must file either married filing jointly or each of you married filing separately. Single is no longer an option. If you file as married filing separately, you may later amend and file a joint return. However, if you file jointly, you may NOT later amend to file as married filing separate.
I assume both of you are calendar year taxpayers and neither of you were a non-resident alien at any time of the year. If either of these are false you may not file a joint return for that year.
Because it is impossible for me to identify and consider ALL the relevant facts, this advice is not intended or written to be used for the purpose of avoiding penalties, and cannot be used for that purpose.