Having sex in general is not safe, my mom was born after my grandpas testes were cut, my dad was born off a condom. Supposably the best time is two days before you start ovulating and two days after you stop, for me and my girl its an 11 day span.
Example: shes starts 23rd over each month give or take and ends on the 30th give or take.
so we stop sexual intercourse on the 21st and begin again on the 1st-3rd of each new month. Again there is no sure way to prevent pregnancy other than absitanance. look up this method on google its pretty well known.