It is most likely from the 1st, if you conceived on the 6th. Sperm can live inside you for about 5 days under good conditions.
If your conception date was reached in a reliable way such as an ultrasound in your first trimester, and if it is December 6, that is when you got pregnant. If the only sex you had near that time was either December 1 or December 14, sperm from the event on the 1st would be the most likely candidate. (Unless there is something about the initial assessment of your date that is off. An ultrasound much later than the 11th or 12th week can be off by a few days, for example.)
Oh and my due date is Aug. 29th