Preheat oven to 325 degrees. Spray an Ugly Sweater Cake Pan with vegetable pan spray.
In a large bowl, beat cake mix, eggnog, oil, eggs and nutmeg with a mixer at low speed 30 seconds. Scrape bottom and sides of bowl; beat at medium speed 2 minutes. Spread batter evenly into prepared pan.
Bake 39–42 minutes or until toothpick inserted into center of cake comes out clean. Cool in the pan on a cooling grid 10 minutes. Remove from pan; cool completely on grid.
With a spatula, ice the cooled cake. Using tip 233 (optional) and green icing, pipe a tree on the cake. Using tip 46 (optional) and red icing, pipe outline neckline, sleeve cuffs and bottom sweater band. Attach sprinkles.