once in a blue moon

When you search “once in a blue moon” (in lower case), Google Calculator will return the mathematical value of “once in a blue moon” - 1.16699016 x 10-8 hertz (or approximately 2.715 years).

Rarely, once in a very long time [...]. This term is something of a misnomer, because an actual blue moon - that is, the appearance of a second full moon in the same calendar month - occurs every 32 months or so. [Houghton Mifflin/Answers.com]

[via SonnySideUp]