First there is no safe days in the menstrual cycle to have intercourse, but some days present higher risk than others.

the abstinence on specific days of the menstrual cycle can be a good practice to avoid pregnancy but not a guarantee because you can easily have late or early ovulation.

The days between the day 6 and day 21 are the ones to avoid having intercourse on.

P.S.: there is also a new invention called Condoms.