If this is going from your LMP, which it likely is, a rough estimate would be about the first week or so of January. Was your last period near Christmas? When you have an ultrasound, dates may change a bit.

Unless you had certain medical procedures to help you become pregnant, it would be difficult to pinpoint the exact day conception occurred.

You have no exact date, I know you said, so many weeks and so many days, if that was an exact date, that would be the day. But due dates are estimates, and are easily off a week, even two weeks, depending on how the estimate was gotten

If you can give me your due date or your LMP I can give you a better answer.

If this is about determining paternity, you will need to have a paternity test done after the birth of the baby.