What would the parents would do with the fall activities?
Frederic and Arianna organize a festival. Corona is a party kingdom, after all. I headcanon that Corona celebrates the equinoxes and solstices, so the festival falls around September 21st.
Captain isn't a particularly celebratory guy, and when he was Captain, he used to be working on festival days. Now, he's enjoying not having to work.
Quirin, on the other hand, is in the middle of his busy season. Even more than Varian, he's in the midst of the harvest. He's got some festive decorations that he pulls out of storage for his stall, which draws people in. He's the primary pumpkin supplier to the kingdom, after all!
Unfortunately for King Edmund, he hasn't celebrated anything in twenty-five years. He's kind of forgotten how. Good thing for him, his son and future daughter-in-law live in the party kingdom and can help him out!