Eggs pop out of the ovary and last up to 36 hours (after which if they are not fertilized, they become non-viable.) Sperm can last up to 5 or 6 days in the woman's body.
If you had sex on the 1st of the month, you could get pregnant on the 1st, 2nd, 3rd, 4th or 5th of the month from that sex on the 1st, if one of those days is the day your egg popped out of the ovary.
If you are trying to get pregnant, have sex every three days from the end of your period to the beginning of the next period (if it comes). That will give you a continual coverage of living sperm in your system to cover whatever day the egg decides to pop out.