I bought this item at the store one-day on auto pilot. I should have looked more carefully. The back of the box shows some of the pics in the calendar. This includes at least one picture that I already did through purchasing The Ultimate Picture Puzzle (2007, ISBN: 1-60320-751-1). (This book is the one that has the surfboards stacked up on the beach on the cover, forest green and lime green colors.) I don't have the other books, but I can only assume that they have also been copied.
Even though I own just one of the books, flipping through the calendar, I did spot about twenty I had already done. And that was how many I counted without looking at every page, so I suspect there's actually going to be more. I know that I won't necessarily remember every difference, but so far I was able to remember a lot of them. Further, I'm now discouraged from buying more of the books, because I don't know which books will contain ones I've already done from the calendar. (That seems a poor marketing choice to me - anger your customers who are already fans of the series.)
Anyway, if you're still interested, then the calendar is a very similar style to the book. Answers are written on the back of the next day's page. Pictures are presented in full color in most instances, along with some black and white ones. There are a few of the pages in which you instead determine how different boxes are put together to form one picture, rather than the standard spot the differences. The pages with spot the differences have two pictures one on top or beside one another and tell you how many differences there are and how much time a competitive person would likely need to complete the puzzle. There's room to mark off your differnces on each sheet. Differences are what you'd expect (slight changes in size, changes in color, changes in shape), some are a lot more obvious than others. Each of the puzzles is ranked by difficulty (i.e. novice, master, expert). Saturdays and Sundays are combined onto one page. The calendar pages are contained within a red stand. I appreciate the fact that it is not a calendar where you tear each page off, but actually you just can pick each page up and then move it to the back (i.e. stacked like a deck of cards). However, I don't really see any reason to keep any of the pages, unless there is one that you can't find all the answers to and want to come back to (it's early in the year and we are on novice, but that hasn't happened to me yet.), or unless you're not writing on the calendar and will pass it along later. You can seem to complete the puzzle with it still in the red stand, but the stand does encroach a little bit on some of the pics, so you might have to remove a few of them to complete them.
It also appears to function fine as an actual desk calendar. The month, date, day of week, and any holidays are written on each page and are easy to read.