Rarely, once in a very long time, as in We only see our daughter once in a blue moon. 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. Further, the moon can appear blue in color at any time, depending on weather conditions. [Early 1800s]

To do something “once in a blue moon” is to do it very rarely: “That company puts on a good performance only once in a blue moon.” The phrase refers to the appearance of a second full moon within a calendar month, which actually happens about every thirty-two months.