You can get pregnant anytime there is a possibility that a sperm can fertilize one of your eggs. Eggs are available for fertilization during and right after ovulation, and sperm can live inside you for up to five or even seven days. So it's possible to get pregnant anytime during a window of about nine days.
Hardly anyone has a completely regular cycle, and younger women usually have more irregular cycles, so it's tricky figuring out when you are the least fertile.