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For a contemporary example, Christmas trees do not feature in English or American Christmas traditions before the 19th century, but were long established by the 20th century. Ditto white wedding dresses.

(The rise of of both traditions dates back to Queen Victoria.)

It's perfectly consistent with it being established in, say, the early 1st century CE and spreading rapidly; a century is plenty long enough for a festival to be "long established", which it was (per the quote) by the second century CE, despite being absent from the literature before the 1st century CE.

It wouldn't feature in the "literature" before then because the oral Torah was not allowed to be written down.

The compilation and writing of the Mishnah was in response to the inevitable upcoming exile.