This seems to be a common mistake in many movies and TV shows. The people in charge don't pay enough attention to details. In a Christmas episode of The Andy Griffith Show, Barney &Andy are in the courthouse discussing Christmas & opening cards. They do mention how it's Christmas eve. In the back of them on the wall is a calendar. It looks like March or April anyway the lettering is a short word. The word December is a lot longer. Wall Calendars do not abbreviate months.