The seasonal events in AWL are more like gatherings than interactive festivals. The residents of the valley get together to partake in an event, but that's about all that happens. You don't get any treats.
Spring 1 - New Years
Everyone gets together up at Romana's mansion between 10:00 am and 9:00 pm. Lumina will give a little piano concert to listen to.
Summer 1 to 7 - Summer Festival
Anytime between the 1st of the season and the 7th, walk up to Gustafa when he is outside of his yurt. The weather must be sunny! Gustafa will then play a little melody on his guitar while everyone gathers around to listen.
Fall 5 - Harvest Festival
This event takes place inside of the Blue Bar from noon until 11:00 pm. Walk into the tavern and Vesta, Marlin, and Celia will walk in behind you carrying lots of food. Ruby works her cooking magic and you all have a feast.
Winter 5 to 10 - Starry Festival
Between 7:00 pm and 11:00 pm, from the 5th until the 10th, visit Gustafa's yurt for another guitar concert. The night must be clear, and this time even Mukumuku will listen in.