With your Seasonal Island Invitation in hand (you don't need to get a new one, but they're still available at Gerolt's if you misplace yours), make your way to the Winter Island, located in the southeast of Mysgardia (it's the spot with the question mark on the map). In there you'll find the Wreth, the Christmas creature now a Winter Seasonal, as well as a brand new creature and several new items. See if you can use the items in Alchemy! I've heard rumours of a new recipe at Akiko's... other shopkeeps also have some new stock, so make sure to have a look around!
This small egg is cold and sometimes you find it someplace else from where you put it.
Also make sure to pay Fawna's Pet Shop a visit - she's been talking about a special Enchanted Plushpup!
All of the above will be available until the end of Winter, at which time new Spring seasonal creatures and items will become available.
(The new seasonal creature was made by FallenAngel101 and Fae, the new Enchanted Plushpup by Rhynn based on the existing ones by Lavie, and the items were made by Dragontamer44722, Fae, ZioCorvid and Rhynn)