every Sunday, a huge number of people all dress up in similar clothes and drive (sometimes an hour+) to gather at a large event space. 

they watch and listen to a much smaller group in front of them, chanting and shouting and waving their arms in the air together.

what weekly ritual am I describing?

helping build farcaster | surfing @ venice, ca | eating @ gjusta / gjelina | “lucky me, lucky mud”

church is the obvious one, which makes me think that can't be it. a livestock auction?

sports, church or depending who you ask, both.