Ill share some ideas for Kingsisle to see that they may respond and hopefully make these into a reality:
St. Patrick's Day: You find and fight Patrick the Leprechaun and earn some St. Patricks Day loot and large amounts of Gold for a limited time.
April Fool's Day: You go on a quest from a Character that is looking for a Lost Treasure in a Cave sending you out to find it and bring it back to them, you go to the Cave and are greeted with "April Fools!" on a Wooden Sign. The Character laughs and tells you how they tried to get that joke to work all day until you had come by, then thanks you for giving them a good laugh.
Thanksgiving: You go around Wizard City collecting Fruits, Flowers and Corn for a giant cornucopia for the residents of Wizard City to have a giant banquet feast and celebrate Thanksgiving, then thanking you the Player for all you have done for the Spiral.