A bunch of authors over on DMs Guild have created a pretty cool unofficial supplement. Book of Seasons: Solstices was written by a long list of contributors including A. Kelly Lane, Alicia Furness, Alison Huang, Ally Sulentic, Annamyriah de Jong, Cat Evans, Collette Quach, Jacky Leung, Jamie O’Duibhir, Jessica Marcrum, Jessica Ross, Jessica Washburn, Kristina Sisto Kindel, Lynne M. Meyer, Ma’At Crook, Miranda Mels, Nemo Bueno, Oliver Clegg, Samantha Lavender, and TK Johnson. There’s a plethora of adventures for any Dungeons & Dragons party to experience with all of theme being themed for either the Summer or Winter Solstice. The adventures are even spread across all character tiers which means there’s an adventure for every group.
You can purchase Book of Seasons: Solstices from DMs Guild starting at $19.95 with physical versions available for more.