The confirmed seasonal events so far are:
- Beach Festival: Summer 25-27 (prep days on 25-26, main festival on Day 27 with Crab Herding, Offshore Racing, Beach Market, and a Bonfire Party at 7pm)
- Chinese New Year / Spring Festival: Winter 25 through Spring 4 (a longer event, with New Year's Eve dinner and fireworks on Winter 28, plus Five Blessings collection, minigames, red envelopes, and a Blessing Exchange Shop)
The Bulletin Board in Starsand Town posts reminders before events. Each event has unique activities, exclusive items, and special NPC interactions. More events will likely be added during Early Access.
Also the Seasonal Events wiki page has a full breakdown of all the activities and rewards. Some of the event-exclusive recipes are really strong buff foods you can't craft normally.