If my due date is december 15, 2014 when did i get pregnant?

Lets review your que: EDD estimated due date is 280 from the date of your first day of your last menstrual cycle. If you know your due date at dec 15 2014, it would appear that the first day of your last menstrual period was March 8. Pregnacy occurs around day 14 of cycle so a good estimated gues would be March 13-17th of 2014. Hope this is helpful/