Ovulation is usually about 14 days before the start of your next period. But because sperm can survive for up to seven days inside a woman's body, having sex even before ovulation can help her become pregnant. The ideal window is three days: ovulation day and the two days preceding it.
Because eggs need to be fertilised within 24 hours after they're released from ovaries, having sex a day or two later may be too late.
However, if you're estimating ovulation day based on the calendar, you might want to have sex a few days before and after the
date to allow for monthly variation.