Regression model to forecast monthly sales

Paper instructions:
WidgeCorp is considering branching out into cold beverages. You have been asked to come up with a regression model to forecast monthly sales of cold beverages for the next year. Think about what variables you might include in your model (you do not need to run a regression and therefore you will not need actual numbers.  You just need to identify and justify your selection of variables that you would include in your model).

Your manager asks for your thoughts.

Respond to anybody who asks you to explain your variable selection (as per the previous question)