sex for the first time and can't decide what to do!!

Okay so according to <a href="">Eve</a> I will not be fertile during the time in which I plan on having sex for the first time with my SO but the next day I will be fertile according the the <a href="">Eve</a> app, would it be stupid to have sex the day before I'm fertile or while I'm fertile even if I use proper protection such as a condom? (Not on the pill or anything) WILL DEFINITELY USE A CONDOM.