Annual festival.

Who knows. There are supposedly "casteless" people, but in reality, it's a caste of homeless people, guys who were kicked out of other castes. But there are a lot of problems with the castes. For example, the castes are supposedly designed for "rationality," to make society more efficient. Children are born into the castes of their parents, but then they take a test that determines which caste they have an aptitude for... but they can freely choose any caste. However, once they've done that, they can't leave it. RATIONALITY. There are "Divergents," people with an aptitude for any of the castes. But it's so irrational to have a smart, brave, honest, peaceful, and selfless person in society, so they kill them. Because of RATIONALITY.