It depends on the length of your menstrual cycle. Ovulation happens 14 days before your period. So if you have a regular 28-day cycle, then Day 1: period, Day 14: ovulation, Day 28: last day before next period if you don’t get pregnant. The chance to get pregnant increases when you have intercourse from a few days before your ovulation to a few days after.
Use a tracker. Lots of hp apps.