"The Latin prefixes "Sept-" (seven), "Oct-" (eight), "Nov-" (nine), and "Dec-" (ten) reflect their original positions in the Roman calendar.
According to this theory, people who continued to celebrate the new year on April 1st, either because they didn't know about the change or refused to accept it, were mocked as "fools." Eventually, this evolved into the tradition of playing pranks and practical jokes on April 1st."

From Vincent Kennedy and CourtZ
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