A: Last night’s celebration was pretty cool. It was my first time to see so many people coming together for a festival.

B: It was indeed a once-in-a-blue-moon phenomenon.

Note: In English, we often use the term “once in a blue moon” to indicate a rare occurrence. This common idiom is often associated with the second full moon in a calendar month, that is referred to as the “blue moon.”

By mid-19th century, the scientist community had concluded that although blue moons were rare, they were real and visible from time to time. This is when the phrase “once in a blue moon” came into being.