Menstrual and plans ..HELP 10 POINTS?

my bf and i are planning to have an picnic on the 4 of july just the two of us..anyway thats when my menstual is due..it will mark the 27th day..for the past few months its been coming every 25-27 days..i definitely dont want to be on because i dislike going out when im on my period because theres not anywhere to change at parks..anyway i have progesterone pills from when my period was going past its normal days and the dr wanted it to stop; so i wonder if i take these pills the day i start my period will it STOP my period? I'm still gonna wear an overnight pad just incase..but the progesterone pills either start or stop periods..so what should i do HELP ME PLEASE..I NEED TIPS