The Christmas decorations can be purchased from the [Decorations] tab in the [Store] during the Christmas period. There are three available items; The Snowman, the Reindeer and the Christmas Tree.
The Snowman and the Reindeer are purely cosmetic items, intended to add some festive cheer to your base. The Snowman also throws snowballs at attackers, however this is again cosmetic and does no damage.
The Christmas Tree does provide a set amount of gems every day, with the gems gradually being provided over time so be sure to check in every now and again to receive your present!
After the Christmas season, the Snowman reverts to his attention pose and the Reindeer takes some time off to become a normal reindeer and wanders around the base. The Christmas Tree also remains but does not provide any more gems. These decorations can be stored and brought out again at any time for free.