Once in a blue moon

Blue moons are so rare, we only see them ONCE IN A BLUE MOON, so to speak. :-)
Is there anything or anyone that you see just ONCE IN A BLUE MOON (very rarely)? 

It was a bit cloudy last night, but did you happen to notice the full moon? Did you know that it was actually a blue moon? There are a couple of definitions for a blue moon, but the original and correct one says that it's the third full moon in a season with four full moons. Each season (spring, summer, fall, & winter) usually only has three full moons, so a season with four is quite rare. Before last night, the last blue moon based on this definition occurred on November 21, 2010, and the next one won't be until 2015. It's a rare phenomenon (event), indeed, which explains the English expression that you've probably already learned: ONCE IN A BLUE MOON (rarely; seldom). 

The other definition, which I, myself, have used to explain what a blue moon is when I teach this phrase, states that a blue moon is the second full moon in a calendar month. Most months only have one full moon, and a month with two is also quite rare. However, I just found out last night that this definition is actually erroneous (a mistake). According to the Huffington Post, a magazine in 1946 mistakenly defined a blue moon by this definition and the explanation simply stuck (stayed; remained). 

Nevertheless, whichever definition you use, a blue moon is a rare event, which is why we say ONCE IN A BLUE MOON to talk about an event or an activity that we do very rarely. Some people cook ONCE IN A BLUE MOON because they prefer to eat out. Others eat out ONCE IN A BLUE MOON because they prefer to cook and save money on food. Some people hang out with friends all the time, while others do it just ONCE IN A BLUE MOON. What about you? What do you do just ONCE IN A BLUE MOON? 

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-- Joe Yu, ESL instructor