Once in a Blue Moon Idiom: Meaning, Origin, Usage & Exercises

In the world of English idioms, some expressions are so vivid and imaginative that they instantly capture attention, and ‘once in a blue moon’ is one of them. This idiom is both poetic and practical, making it a valuable tool for IELTS candidates aiming for higher band scores in IELTS Speaking and sometimes in the writing module. 

In this blog, we will explore the meaning, origin, real-life usage, and IELTS-specific examples of this idiom, followed by challenging exercises to deepen your understanding and boost your confidence.

Once in a Blue Moon Idiom: Meaning

The idiom ‘once in a blue moon’ refers to something that happens very rarely or almost never. It conveys the idea of extreme infrequency, often with a slightly poetic or whimsical tone. 

It is commonly used in both spoken and written English and can describe a wide variety of rare events, from personal habits and social gatherings to natural phenomena or unusual experiences. The phrase is informal to neutral in tone and is especially useful when you want to highlight how uncommon or exceptional something is in everyday life.

Origin of Once in a Blue Moon Idiom

The phrase has a literal astronomical reference. A ‘blue moon’ is a rare second full moon in a single calendar month, occurring roughly once every 2.5 years. Although the moon does not actually appear blue, the term symbolized rare or unusual events as early as the 19th century.

The expression became popular in English idiomatic usage by the early 20th century and is now common in British and American English.

Once in a Blue Moon Idiom Usage

The expression ‘once in a blue moon’, one of the most useful idioms for IELTS Speaking to score band 8.0+, is frequently used in casual as well as semi-formal conversations. Below you will find a list of possible applications and ways to use it.

• He only visits his grandparents once in a blue moon, although they live just a few hours away.
• We eat out together once in a blue moon because we prefer home-cooked meals.
• I watch television once in a blue moon—usually during major sports tournaments.

Once in a Blue Moon Idiom Usage in Scenarios

Scenario 1: Social Gatherings

• Scenario: Your friend invites you to a birthday celebration, and you mention how rare it is for you to attend because of the workload.
• Use: "I usually go to parties once in a blue moon, because of my workload. That is why I look forward to attending this one!

Scenario 2: Work or School Events

• Scenario: You rarely take an off at work.
• Use: "I generally take a day off work once in a blue moon."

Once in a Blue Moon Idiom: Synonyms and Related Phrases

We have listed some expressions and common idioms in IELTS Speaking related to the idiom, once in a blue moon.

Once in a Blue Moon Idiom: Detailed Usage in IELTS Contexts

Whether you are discussing your hobbies in Part 1, narrating a personal experience in Part 2, or expressing opinions on modern lifestyles in Part 3, using idioms like ‘once in a blue moon’ can help your speech sound more natural, fluent, and idiomatic, just what IELTS examiners look for in the speaking test. 

IELTS Speaking Part 1 (Topic: Habits and Hobbies)

• Question: Do you enjoy going to the cinema?
• Answer: To be honest, I don’t go to the cinema very often. I usually prefer streaming movies at home because it’s more convenient and comfortable. Maybe once in a blue moon, I’ll visit the cinema if there’s a film I’ve really been looking forward to, like a major release or something visually spectacular that deserves the big screen experience. But overall, it’s quite rare for me.

IELTS Speaking Part 3 (Topic: Social Interactions)

• Question: Do you think people socialize less now than in the past?
• Answer: Yes, I believe people do socialize less face-to-face compared to the past. With the rise of digital technology and social media, a lot of interactions have moved online. While this makes it easier to stay in touch, it often lacks the depth of real-life conversations. Nowadays, people are so busy with work or glued to their screens that meeting friends or family in person happens only once in a blue moon. In earlier times, socializing was a regular part of life, but now it seems like a luxury.

Once in a Blue Moon Idiom: Practice Exercises

Below you will find a series of exercises created to improve your comprehension and application of the idiom ‘once in a blue moon’, allowing for a better understanding of its meaning and usage in different contexts.

Multiple-Choice Questions

1. What does the idiom "once in a blue moon" mean?

a) Frequently

b) Never

c) Rarely

d) Always

2. Which sentence correctly uses the idiom "once in a blue moon"?

a) She eats chocolate once in a blue moon because she loves it so much.

b) He visits his grandparents once in a blue moon since they live nearby.

c) They watch movies once in a blue moon because they have a lot of free time.

d) I go hiking once in a blue moon because I rarely get the chance.

3. If someone says, "I only cook fancy meals once in a blue moon," what do they mean?

a) They cook fancy meals very often.

b) They never cook fancy meals.

c) They cook fancy meals rarely.

d) They always cook fancy meals.

4. Why might someone say, "Opportunities like this come once in a blue moon"?

a) The opportunity is very common.

b) The opportunity is very rare.

c) The opportunity happens all the time.

d) The opportunity never happens.

5. "My friend only calls me once in a blue moon." What does this suggest about their calls?

a) The friend calls very frequently.

b) The friend calls very rarely.

c) The friend never calls.

d) The friend always calls.

Once in a Blue Moon Idiom: Answer Keys for Practice Exercises

Now, compare your responses to the answer key provided below to determine your understanding of the idiom ‘once in a blue moon’.

1. c) Rarely
2. d) I go hiking once in a blue moon because I rarely get the chance.
3. c) They cook fancy meals rarely.
4. b) The opportunity is very rare.
5. b) The friend calls very rarely.

To conclude, the importance of idioms like ‘Once in a Blue Moon’ in the IELTS exam cannot be ignored! It is a vivid idiom that enhances your ability to describe rarity or infrequency with style. So, use it wisely in Speaking Part 1 and 2 or IELTS Speaking practice tests to make your language sound confident and fluent.

Useful Links:

• Help and Encouragement: Useful Idioms for IELTS Speaking
• Home away from Home Idiom: Meaning, Origin, Usage & Exercises
• The Gift of the Gab – Idiom of the Day for IELTS
• Cut Things Fine - Idiom Of The Day For IELTS 
• 10 Useful IELTS Speaking Tips to Impress the Examiner
• 50 Recent IELTS Speaking Topics 2023 Part 2 & 3 with Model Answers
• Recent IELTS Topics in IELTS Speaking Tests 2025