The Challenge Festival Summon is available during the Challenge Festival Event. It includes Heroes from all previous Challenge Events and some of their Costumes. The Summon Gate does not include Classic (Season 1) Heroes. There is also a chance to get the current Hero of the Month as a bonus draw.
The Challenge Festival Summon includes a Festival Bonus Chest that is awarded for every 10 summons. You can claim a maximum of 9 chests during the Summon Event. Chest progress will carry over to the next Challenge Festival Event.