once in a purple moon

By analogy with once in a blue moon. First use appears c. 1924. See cite below.

• "However, in spite all of our complainings, there was a distinct pleasure in shopping along this street, because once in a purple moon, one did strike a really nice model and so very reasonable, that one wondered how the manufacturer could afford to bring it out at the price."
• "Once in a purple moon, it may be necessary for the top men to settle some awful impasse between themselves."
• "Again, these usually show up in major currency auctions, only once in a purple moon would a dealer have one in stock."

• rarely
• seldom
• infrequently
• scarcely
• seldomly
• uncommonly
• unoften

• blue moon