We all know the phrase – if something never happens, or very rarely, it happens once in a blue Moon. But what is a blue Moon, what does the term mean and how often does a blue Moon occur?

Our modern calendar is rooted in astronomy: a day is based on the rotation of the Earth on its axis, a year comes from the time it takes Earth to orbit the Sun, and a month is based on the revolution of the Moon around Earth.

For the most part, the lunar cycle and our calendar are in sync, the Moon waxing and waning each month, giving rise to the phases of the Moon, with about 29.5 days between full Moons.

For more practical lunar guidance, read our guide on how to observe the Moon, or get lunar dates, times and phases delivered directly to your email inbox every week by signing up to the BBC Sky at Night Magazine newsletter.

Typically, then, we see 12 full Moons a year, one on average falling neatly into each calendar month.

But the two calendars don’t precisely match up. A calendar year contains around 11 days more than the number of days in 12 lunar cycles, so eventually the difference makes itself known.

It’s then that we get two full Moons within a calendar month, and this has led to the informal meaning: a blue Moon is the second full Moon in a month.

What’s a seasonal blue Moon?

You’d be forgiven for thinking the definition of a blue Moon as the second full Moon in a month dates back centuries, but it’s actually startlingly modern: a 1946 edition of Sky & Telescope magazine.

Writing for the magazine, James Hugh Pruett misinterpreted blue Moon dates in old copies of the Maine Farmer’s Almanac periodical. This astronomically incorrect definition is now arguably the most widely-used understanding of a blue Moon.

But the older, more traditional meaning is a ‘seasonal blue Moon’ and is specific to the astronomical season.

An astronomical season is between a solstice and an equinox. In the northern hemisphere, for example, the summer solstice (around 20–22 June) denotes the start of astronomical summer and is followed three months later by the Autumn equinox (around 21–23 September), the beginning of astronomical autumn.

Within that time, commonly we would see three full Moons.

The original Maine Farmers’ Almanac definition says a blue Moon is the third full Moon in a quarterly season of four full Moons.

We’ll get one of these seasonal blue Moons on 22 August 2021.

How often does a blue Moon occur?

Like many idioms, “once in a blue Moon” is not especially accurate. Blue Moons occur relatively frequently, every 2–3 years.

Going by the term meaning the second full Moon in a month, there was one blue Moon in 2015, there were two in 2018 (in January and March; February had no full Moon) and one in 2020 which, as it fell on 31 October, was predictably named a Halloween blue Moon.

Looking ahead, there will be monthly blue Moons on 31 August 2023 and 31 May 2026.

When are the next blue Moons?

Second full Moon in a month:

• 31 August 2023
• 31 May 2026

Third full Moon in a season that has four:

• 22 August 2021

Is a blue Moon actually blue?

Many people may think this barely needs pointing out, but no, a blue Moon is not usually blue and the term 'blue Moon' does not refer to the colour of the Moon itself.

The appearance of a blue Moon will be no more or less affected by atmospheric conditions than it is in any other month, which is to say it will appear in shades of white, yellow, grey, or possibly orange and reddish (such as during a lunar eclipse).

This comes down to the light that the Moon reflects passing through dust in our atmosphere as it travels towards us here on Earth. Depending on the size and density of those dust particles, the light we see will appear to vary in colour.

But it is highly unlikely to look blue as the necessary atmospheric conditions are rare.

The eruption of Krakatoa, Indonesia, in 1883, which killed 60,000 people, was one such occasion.

The devastating explosion threw up plumes of smoke and dust, including particles 1 micron wide, a size that efficiently blocks red light but allowed blue light through.

As a result, the Moon at night appeared bluish for several years after the eruption.

Apart from such rare occurrences, the Moon is only likely to look blue in a photo where the photographer has used a coloured filter or added the blue in processing.

Where does the term 'blue Moon' come from?

The English language is bulging with phrases and sayings based in our ancient connections to the seasons, the land and the sky. But, despite its common usage, the roots of 'blue Moon' and 'once in a blue Moon' – linking the unusual extra Moon and blueness – are not clear.

One early mention of the Moon being blue is in a 1528 pamphlet. It describes untrustworthy Roman clergy as “wiley foxes” who will say “the Moon is blue” (“the mone is blewe”): that is, will expect people to believe any absurd lie.

Scroll forward to journalist Pierce Egan’s 1821 bawdy bestseller Life in London, and 'blue Moon' had come to denote ‘unusual’ and ‘rare’:

“Their attention was at this moment attracted by the appearance of two persons dressed in the extreme of fashion, who, upon meeting just by them, caught eagerly hold of each other’s hand, and they overheard the following—’Why, Bill, how am you, my hearty?—where have you been trotting your galloper?—what is you arter?—how’s Harry and Ben?—haven’t seen you this blue moon.’”

Now, from schmaltzy songs, football chants and movie titles, to Jeff Bezos’s lunar lander, you’ll come across ‘blue Moon’ exceedingly frequently (certainly not once in a blue Moon).

Jane Williamson is a science journalist and writer.