once in a blue moon

See blue moon. First use appears c. 1833. See citation below.

• "We are no advocates for the eternal system of producing foreign operas to the exclusion of the works of English composers, but once in a blue moon such a thing may be allowed."
• "[T]ime expanded and space contracted, and the name Blue Moon took on a symbolic meaning, as if the future, so delicately plausible, were of a kind that might happen once in a blue moon only."

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

• never event
• once-in-a-lifetime