According to the American Pregnancy Association, it's almost impossible to know the exact date of conception unless a woman has undergone artificial insemination or in vitro fertilization. Typically, for women with regular periods, conception usually occurs within 11-21 days after the first day of their last period.
For women who have been tracking their ovulation dates, the website JustMommies offers a potential conception calculator. The luteal phase, or second half, of a woman's period is generally 14 days regardless of the length of her period. Subtracting 14 days from the length of her cycle pinpoints her ovulation date. Conception likely took place either the day of or the day after ovulation.