if you google babys due date you should come up with the results to find out when the baby was concieved. depending on your cycle, you can almost count back to when you were most fertile. A doctor would be able to tell you best.

i would ask your doctor for an exact date if you must know. does it really matter, though, which day you conceived? those two dates are right next to each other, so it's possible you conceived on either date.