High school reunions happen every 5 or 10 years. Typically the first reunion is 10 years after high school has ended, and then every 5 or 10 years after that. For example, it may happen at 10, 20, and then 25 years. The physical invitation, email, or Facebook invite will list the exact reunion date.
Most people attend their high school reunions, first and foremost, to reconnect with their former friends. Expect that old cliques will re-cohere; that the in-crowd will hang out together and catch up with each other.