I see this question often on comments or reviews of 17- and 18-month planners, and I'm always surprised by the question. Personally, I like to plan as far ahead as possible in my planner so more weeks is always better.
17 month planners begin in August, and 18 month planners begin in July. Both go all they way through December of the following year. These types of planners are very popular and many people prefer them.
But, some people argue, why not have a 12 month planner that begins either in August or January? Those extra months are just going to go to waste.
Well, I disagree.
First, the reasoning behind a 17- or 18-month planner is that you can start the planner at the beginning of the academic year and plan all the way through the end of the following calendar year. These planners are often geared toward students, and moms of kids in school. It's more convenient to schedule the entire school year in one planner.
So then why the extra months? Why not just have the planner end in July and start a new one again in August?
Well, there are several reasons:
Some people really do like to plan all the way to the end of the next year.
Some people (like students who will graduate at the end of the school year) need to transition to a January-December planner
And, some companies only make 17- or 18-month planners and don't have separate academic-year and calendar-year versions. With both the academic and calendar years covered, they have a wider potential customer base.
To those folks who say the unused months are wasted, I say, never!! Those pages can be used as Notes pages, and don't we all want more notes pages in our planners??
**Edited to add: now that we are almost at the end of the school year, I am realizing just how valuable an 18 month planner can be. As our after-school activities are ending for this term, they are being scheduled to resume in August, some at different days/ times than they currently are. If I were using just a 12 month academic-year planner, I wouldn't be able to schedule these new times for the autumn term. Being able to plan the entire school year in one planner, and then plan the start of the new school year in the same planner, is hugely valuable!
Do you use a 17- or 18- month planner? Why?