The same question has been asked on StackOverflow Meta. Here is the answer that was provided there:
That started with the user Olafur Waage who lives in Iceland. I believe it started with someone asking a Friday type of fun question and it started as a comment, then turned into a tag. And then people started slapping it on more fun meta questions.
Friday is kind of the official "let a few fun questions survive on meta" relaxed day. I believe someone asked a fun type of question and it was not Friday and someone's defense was it is always Friday in Iceland!
