Nothing like a fresh bit of Shirow every month, eh? Shirow's art has been featured on calendars for quite some time now, dating back to 1988's Appleseed calendar. While it wasn't a regular phenomena at first, there's been at least one Shirow calendar every year since 1998. For the most part, we've been seeing them from Shirow's primary publisher, Seishinsha, but we've also been getting them from Dark Horse the last few years. Unfortunately though, we're left without a Japanese calendar here in 2005, as there seems to be some sort of shift in Shirow's relationship with Seishinsha and Kodansha, though the details are unknown. What this means for future Japanese calendars is anyone's guess, but hopefully we'll get one next year.
For quite some time now, Shirow's art has been featured in yearly calendars from Seishinsha. While I think some of the pieces in the calendars are original commissions, I'm fairly certain that nearly all of the come from other projects, most notably the massive amount of cover art he's done for Comic GUM magazine, but also drawing from other commission work he's done. Additionally, most of the art in the calendars has also been reprinted in the Intron Depot artbooks, which makes things easier for the less hardcore collector that can't find the calendars for themselves.
For the last few years, Dark Horse has been issuing a Shirow calender of their own, made up of various images they've gotten lisence to use. As such, anyone following what Shirow does in Japan isn't going to be seeing anything new in these. In particular, Dark Horse uses images from the Intron Depot artbooks, as they already have license to the art in them, though it happens that nearly every image they've used so far has also shown up on the Japanese calendars already. Worse, where the Japanese give 13 different images, with a unique cover, each of the cover images on the Dark Horse calendars also appears as one of the months in the calendar. Still, they are nice because it's an English calendar, and they're printed in a much larger size than the Japanese ones.