To start off we have the Joker getting the infamous carnival.
Note the rain.
Booster arrives at a different spot but at the same carnival. Now this would have to be right after Joker makes the "purchase" and gathered the freaks but before he visits the Gordons. I'm guessing he has some time to spare since he spends a good deal of it inflicting pain onto "Golden Rod." (That will always sound like either a C3P0 reference or something very perverted.)
The Joker and his misfit gang inflict various types of pain onto Booster, partly off panel including a roller coster racing at him while chained. When their done with him for the moment Skeets and him break free to try to make their way to the Gordons.
Despite their intrusion (and various attempts later on) Booster can not break the loop of events. Things are changed to based on his presence like him fighting the Joker but he can not stop what was shown in the Killing Joke at all.