They aren't set in stone. The better you do, the more chances it'll throw a firefly/hunter attack at you. You might still get one even if you do poorly, but generally the game will throw more pop increase events at you. So you can easily go through a few of the last weeks without even meeting one until week 12, which is the only real set in stone one.
I think week 6 is also one that is set in stone, so the game can tell you 'I'm throwing the kiddy gloves down now'. Not sure though.
-Memento Mori is a latin phrase that can be freely translated as 'remember you are mortal' 'remember you will die'. -Cogito Ergo Sum, Je pense, donc je suis.