I've been active for a few years and I don't remember seeing anything like this in the past. Is this a first for Stack or did I just miss them in the past?
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Depends on what you mean, but you can check out the Stack Exchange promotion archive for cool events that have happened in the past. Winter Bash is just an extension of Arqade's Hat Dash from last year, for example.
April Fool's Day is also usually good for a laugh around here.