Once upon a time in old France there was a small vineyard run by a coven of witches.
The grapes that grew there were used exclusively for the production of raisins, and were always harvested under a full moon to preserve the magic properties some of them had.
The witches believed that one full moon in seven was a bad one, and while the raisins made from grapes harvested under a good moon were ordinary (albeit of high quality) and mostly went to general consumption (witches have to make a living too, you know).
Grapes harvested under a bad moon would rot - with one exception.
The biggest and strongest of the grapes would survive to become the legendary.... "Bad Moon Raisin".