The rarity scale is a less-than-scientific attempt to give collectors an idea of the scarcity of each calendar. I have based it primarily upon the number of eBay auctions in which a calendar has appeared but adjusted by the availability at various on-line booksellers, the number that I know of in the hands of other collectors or collection (e.g. - Marquette University), and the number that were created (as for the various issues of the Tolkien societies) when I have able to determine same.
10 The rarest because of age, number issued, and/or condition (referring to those still mint in sealed mailers/envelopes or shrink-wrapped). Usually, I know of less than five examples of it.
9 Scarce, not impossible to collect, but most are in the hands of collectors. Usually, I know of less than 10 examples of it. Unless one appears on eBay (on average: less than once per year), you have to know someone who knows someone who has one (and is willing to part with it!)
8 May appear on eBay on average once or twice a year, or fifty or fewer were created.
7 May appear on eBay on average less than once per month, or 100 or fewer were created.
6 May appear on eBay on average one or more times each month, or 200 or fewer were created.
5 May appear on eBay on average less than once per week, or 1000 or fewer were created.
4 May appear on eBay on average one or more times per week, or more than 1000 of them were created.
I have not rated any calendars at 3, 2, or 1. Basically, these are the current issues of the commercial publishers (e.g. - HarperCollins). You can find them "everywhere" today. But who knows in ten years or more?