The boss battles, classic tournaments, team battles, and song votes seem mostly to be random. (The Weekly tournament, however, comes every Sunday at the same time. In the US, it comes around noon eastern time.) Perhaps the dancers who happen to be on the dance floor at any given time influence what happens to some extent; that is, it may not be completely random. However, they don't follow any regular schedule, other than the weekly tournament and happy hour.
There are usually 3 different bosses each coming several times per day for about a month, and then the theme changes and new bosses come out. The Halloween bosses have already come and gone, so you'll probably need to wait until next October to see them again. The current bosses will probably be around at least until Christmas, so if you still need one or two of them, you should have plenty of opportunities. So far, the bosses seem much more reasonable to beat in JD 2016 (I've participated during about 40 wins, and I think I've only been around once when the dance floor lost), whereas in JD 2017 there were a few bosses that were only rarely beaten.
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