There are a number of drinks that are seasonal — they are available only at certain times of the year. The Pumpkin Spice Latte is available in the fall, for example. The Gingerbread Latte, Eggnog Latte, and Caramel Brulee latte are available only during the winter holiday season.
It's partly to keep things interesting, partly due to ingredient availability (getting eggnog in summer is much harder), and partly so the ingredients don't waste space behind the counter during times of the year when they don't sell well.
Some seasonal beverages go on to be available year-round. In some areas, Peppermint Mochas can be ordered any day of the year. Cinnamon Dolce Lattes used to be a late winter drink, but are now available any time.