File 539 and ask for one week of extension. Plenty of time. As long as you have the filing records (application and INS receipt), you are fine to leave on the 12/8. Many people ask for extensions of months. Even if they get denied, INS would give them additional time (after the visa expiration time) to leave the country.
In your case, you are asking for one day. I would file the 539 today or tomorrow.
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