How To Calculate The Exact Day Of Conception?

hi i need help figuring out when i conceived.I unprotected sex in july 09 but can not remember which date i then had sex wiv my boyfriend in the early hrs of 10/08/09 during which the condom got stuck inside me i then done a pregnancy test on 24/08/09 which was negative & then done 1 on the 01/09/09 which gave me a positive reading can you please help me as not knowing is eating away at me thank u