Challenge events occur once a month on a rotational schedule. There are typically 3 tiers of difficulty: Rare, Epic, and Legendary. Each of these tiers only allow heroes of that rarity to be used. The rewards you can earn are based on your Event Score.

Challenge Festival II4 days
Clash of Knights5 days
Challenge Festival4 days
Mighty Pets4 days
Sanctuary of Gargoyles4 days
Slayers of Fell Shadows4 days
League of Villains4 days
Starfall Circus4 days
Tower of Magic5 days