No special software feature here, certainly no extra installation, just one hell of a lot of setting up templates to do whatever people have made those calendars capable of doing. I have literally no idea why they are so complicated, and couldn't immediately find anywhere explaining how they work; but presumably *someone* knows - maybe User:ABCD, who seems to have created some of them, could tell you...
I couldn't work out how it was done from the wikipedia. sounds like it's more trouble than it's worth to me (I thought there might be some kind of installer, perhaps). thanks.