I'm looking into Botox for an upcoming gathering. How long does the effect last?

I have an important alumni get-together next month, so I'm considering getting a light Botox injection in my forehead before then. However, my biggest concern is that it might look unnatural and obvious after the procedure. It would be embarrassing to meet friends with a stiff face. By the way, how long does the firming effect usually last? Also, I'm waiting for realistic advice from those with experience on how many days before the gathering would be best to get the injection so that the swelling goes down and it looks natural!