My due date is on 28th December 2012,when did I get pregnant?

Sometime between your last period and the one that was missed.

Sorry that is about as exact as you get. There is no exact date on conception, it is at best give or take a week or two.

When was you last period, do you have dates of sexual contact during those next @ 4 weeks.

But it really never matters when you conceive and of course even delivery date is a estimate and seldoms comes that exact day, Babies have their own schedule.

When a sperm fertilized your egg. Since we weren't there to watch that happen, your actual conception date is guesswork.