At the beginning of the game, festivals won't be available. Jarvis will eventually tell you that he feels inspired to research some of the festivals that would occur in Clover Town. The festivals are random occurrences. You may spend weeks without a festival in-game and then suddenly have the same two within 7 days of each other.
If you have triggered a festival, you will be notified by the festival master of ceremonies as you exit your house when you start playing for the day. The festival MC will wait for word of your participation by your shop.