A: My old schoolmate John has come up with a business idea and asked me to join him in his startup as partner. What do you say?

B: John is a brilliant young man and the odds are he will be successful. It’s an opportunity that comes once in a blue moon and you should take his offer.

Note: This idiom means “once in a great while, rarely.” A seasonal blue moon is the third full moon of an astronomical season that has four full moons. A monthly blue moon is the second full moon in a calendar month with two full moons. Since there are normally 12 full moons a year (one per month), blue moons only occur about once every three years. It became a convenient measure of a long period of time and gave rise to the saying.