After you visit the Dreaded Den with Kieran, it's time to go to the Festival of Masks.
Explore the Festival
If you go to the candy apple stand that Kieran went to and talk to the shopkeeper, there will be a cutscene.
If you go north, then northeast to the corner of the fence, you can talk to the girl there to learn the Shhh! emote. You can use it when you use the camera in Selfie Mode.
Go west from there, and you can go south from the path to find a kid who will teach you the Please! emote. You can press up on the directional pad to choose it from the emotes list.
Play Ogre Oustin'
Go to the building north of the path to find Kieran. Talk to him and there will be a cutscene. Say Yes and you will play Ogre Oustin' for free. Carmine challenges you to beat her high score of 6390. To play, ride your pokémon through as many balloons as possible to collect berries. You can only carry a certain number of berries, so if you have a lot of them, you will need to go back to the tables before you can collect any more. Then take the berries to the tables and check on each table to drop off the berries. Pokémon will start running toward the tables to eat the berries, so be sure to go back to the tables and press R near the pokémon to scare them away if they are eating your berries. There are three stages. Afterward, you can get rewards like Mochi, Mints, and other items.
As a reward for your first time playing Ogre Oustin', you get an Exp. Charm, which permanently increases the Exp you get from battles. From now on, you can talk to the person behind the counter to play Ogre Oustin', and now you can play with others locally or over the Internet, or you can play alone. Clear Easy again to unlock Normal, then clear Normal to unlock Hard. The first time you complete Hard mode, you get a shiny Munchlax.
Investigate the Strange Sighting
After you completed Ogre Oustin', you noticed something strange. When you're ready to move on, go west to the gate to Oni Mountain, and there will be a cutscene. Then go up the path toward Oni Mountain, and there will be another cutscene.
You won't be able to leave Kitakami Hall until you talk to Kieran, so if you're ready to leave the festival, go east through the area, then south to the Kitakami Hall south entrance, and talk to Kieran to say you're ready to leave.
After the Festival
Your pokémon will be healed up and you will be back in the Community Center. Go outside when you're ready. Carmine asks you to meet her at her house, so go there and talk to Carmine's grandfather. After the cutscene, choose the Teal Mask from your inventory. Then, when you're ready to hear the story, talk to Carmine's grandfather again.
Afterward, go Peachy's shop, where you will find Kieran. Talk to him and there will be a cutscene.
One way to reach the final sideboard is to go west from Mossui Town, then west through the Apple Hills until you reach the Wistful Fields, where you can go clockwise around the mountain to reach the destination marked on the map.