It's from the movie, "The Proposal." Two persons are talking about their coffee. "So you drink unsweetened cinamon light soy lattes?" "I do. It's like Christmas in a cup." Will you explain why this person says "Christmas in a cup" ?
Cinnamon is a traditional flavoring in Christmas cakes, and ,as far as I know, most americans think about this beautiful cozy holiday when smelling or tasting cinnamon. Therefore, latte (coffee) with cinnamon in it brings memories of Christmas - it is Christmas in a cup.