The mind-rain isn't mentioned by name in Specials, but it takes place towards the end of the book.
The mind-rain took place three years before the first chapter of Extras. Mind-rain is the name for when most of the pretties took Maddy's pills to cure their brain lesions.
After the Mind-rain, face-rank and merits were invented. Many social rules from before, such as pretties not being allowed to hang out with uglies, were removed. The meaning of pretty became somebody famous, and Tally Youngblood was the most famous person in the world.