An event that only appears in the Main Chapters. You have a double chance to get treasure in certain stages during this event. This event usually lasts an hour. This happens at least once a day, except for Cats of the Cosmos, which doesn't have a set time for treasure festival. So, when you see Cats of the Cosmos having treasure fest, you should go and farm some treasures!
They will usually come for a single stage at a time, however, rarely all stages can have a treasure chance! Magic!
Sometimes, during a big celebration event, Treasure Festival will be on, celebrating the event. This event usually lasts the entire day.
PONOS Event Calendar
Here is a link to the PONOS Event Calendar where you will find daily schedules for all events that appear on a rotation in Battle Cats, including treasure festivals and event stages that appear at a particular time of day. The calendar updates every Monday and lasts through the following Sunday.