When can I take a test?

Sami

If my SO and I had sex on all the highest fertility days when should I take a test? I am supposed to start my period in 7 days but I didn’t know if I could take a test before the day I’m supposed to start my period. And if I am pregnant would I miss this month period or next months?? Sorry I feel dumb because I’m so uneducated. 😕🙄