No. Nothing says that you have to focus on a particular month. However it is traditional, especially in the younger grades, to focus on the time of year and seasons as this helps to enforce it with younger children. Some teachers focus on books of the month instead but they still usually put the name of the current month up for students to reference. Having a sense of time and the point of the school year being experienced is important to students. But if you can figure out another way to create the same motivation using another technique, feel free to share it with us!