Since the 12th month is December, a 13th month should obviously be called Undecember (the undec- prefix meaning 11 in Latin).
Better yet, just stick the new month in the middle of the calendar, but don't rename the months that have numbers in their name. It already worked once (thanks, Romans).
After the revolution, the French came up with some poetic and meaningful (if you live north of the Tropic of Cancer) names for months. We could use those.