I'm confused, "first day"? Wont you start in only a month? Pretend everything is ok and announce you are quitting just on the first day, that would be unprofes

I'm confused, "first day"? Wont you start in only a month? Pretend everything is ok and announce you are quitting just on the first day, that would be unprofessional. Confirm and sign with the other company and inform that you will refuse the offer of the first one as soon as you have this confirmation. That's professional.