If you start looking at planners you will soon see that there are two categories:
Dated are printed for a specific 12/6/3 month span, while undated require you to fill in the dates yourself.
I have used both in the past, while I currently use dated ones I think there are good sides about both.
What I like about dates ones is that they come ready to use.
While undated ones have the advantage that you can start using them no matter when you feel like, and they are much easier and cheaper to produce.