It's usually a notification that it's going on and if one of my Sims has a BF/GF I usually invite them to go and if a random friend shows up later, so be it.
This is one of my least favourite festivals as either my single, young adult Sim will go along only to find the entire festival is populated by adult townies who are either a) already married b) freakishly dressed or c) a bad combination of erratic/evil and hot headed. I suppose that reflects real life speed dating....
If a couple go then I have to watch them like a hawk! What should be a nice, romantic evening out for them to get their romance going can soon turn into a flirt-fest with all kinds of random townies. Cue anger, a drop in romance and friendship levels and a huge amount of relationship repairs afterwards. I try to stomp on any interaction with Townies if I want my couple to have a successful night out.