Okay, so you can't use tampons (as I saw on an earlier comment/answer), so the best you could probably do is get period swimwear. If you're new to your period or relatively young (about under 12 or 13), that should do it for you and you shouldn't have to worry (as long as it's a quality one, anyways)! If that's not an option, just tell your friend it's a little up in the air and your not sure yet- you can just cancel the day of if you do happen to get your period that morning, or just bring pads with you when you go in case you get it there! :)
