Select the most appropriate meaning of the given idiom.

Once in a blue moon

Very rarely

The most appropriate meaning of the given idiom "once in a blue moon" is very rarely.

It is an idiom that is used to describe something that happens very infrequently. The idiom is thought to have originated from the fact that a blue moon is a second full moon in a single month. This occurs about once every 2.7 years, which is relatively rare.

For example, you might say "It snows once in a blue moon in this city." This means that it snows very rarely in this city.

The other options are not accurate descriptions of the meaning of the idiom.

• Blue sky: This is the clear sky without clouds.
• On a full moon night: This is a night when the moon is full.
• At one time in space: This is not related to the meaning of the idiom.