It seems to be created like any other made-up cultural tradition, and as such the "strength" of the holiday is based on how many people consider it legitimate. So major religious holidays tend to be pretty huge, whereas Talk Like A Pirate Day is probably limited to niches of the Internet (FOR NOW, we must spread the word).
As for initial creation, I dunno, probably based loosely around an actual event. I haven't done much in-depth study of all holidays though. I imagine some of the older ones are so warped from their original intention that it doesn't even make sense anymore.
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