Conception date?

So im confused as how this works lol. Might be weird but i would like to know around the date my baby was conceived since i know my 1st babies conception date&i think its something beautiful to remember(cheesy i know lol) so my mirena was removed August 31st i had my period from 08/01- 08/04 and no period after that. I make 21 weeks this friday. Im confused doe since ppl say it wasnt really a period. Advice?