When buying an agenda book, you really need to take a look at the time of year first.
Seriously, because there are, these days, very cute planners available for both the calendar year and the academic year (that's summer to summer).
Student planners, like U Chic, are of course academic day planners: The start July or August and end July or August of the following year. Some give you a few bonus months in the second year or even last all through December.
Moleskine 18 month planners do that: they start in summer, but last all through the next calendar year.
Then there are the mom planners. They are also usually 18 month planners, starting in summer, but they have extra planning room so you can keep track of not just your own schedule, but that of the whole family: husband, partner and kids.
When buying a planner in summer, I would really recommend you go with an academic date book.
However, if you're set for the current calendar year and want to get ready for the next one, you should of course pick a day planner that covers the calendar year.
Peter Pauper Press comes out with a beautiful line of planners for that purpose. I can't decide which I like best - the pink blossoms, the Asian Peonies or Paris (Bon Vivant). Can you?
Of course, these cute agenda books make a great gift as well.