I have Farmer's Market Day (more or less an Earth Day substitute) on the first day of summer. I don't have the game open right now, but I think the activities are mostly gnomes and gardening.
Summer Vacation in mid-summer: something like travel, BBQ, water activities, fire, fireworks, tell stories. It depends, but it often takes the form of having a cookout in the park with friends and family. Then Mountain Day in early fall: travel, BBQ, fire, tell stories. It's on a Friday, so I usually have the household take a long weekend... often to Granite Falls, but sometimes another destination, but it's always sort of camping/hiking focused.
Parents Day is a Monday in spring, sometime after Bunny Fest. Grand meal, holiday ceremonies, give gift, give flowers, tell stories. Pride Day is... I think... late spring? Invite guests, party spirit, have drinks, tell stories.
Oh, also, I kept Winterfest but also added Winterfest Eve, which I think is baking, fire, decorate, and tell stories? Maybe also holiday ceremonies.