Is the day right after your period the time to have sex if you want to avoid pregnancy?

That is reasonable: You are most fertile about 2 weeks after the first day of your menstrual period, as that is typically when you ovulate. Thus, having intercourse on day 7-8 should be a safe time. However, every woman has her own cycle, and this may be different for you. The best way to avoid pregnancy is either to abstain or use a reliable method of contraception.