This episode is part of the Avatar Book Two - Earth
Avatar The Last Airbender Episode Guide ~ via Wiki
After getting attacked by a band of Fire Nation soldiers, the gang stumbles upon a town celebrating Avatar Day. However, they find out that this is an “Anti-Avatar” celebration, and Aang is arrested for killing Chin the Great in a past life. Sokka and Katara go to Kyoshi Island to find clues to prove Aang’s innocence.