Select the most appropriate meaning of the given Idiom:-

Once in a blue moon 

Something that happens very rarely

The most appropriate meaning of the idiom "once in a blue moon" is something that happens very rarely. The idiom comes from the fact that a blue moon is a second full moon that appears in a single month. This happens about once every 2.5 years, so it is a very rare occurrence.

The other options are not as accurate descriptions of the meaning of the idiom. In the evening is not a common expression for something that happens very rarely. The blue colored sky during on a clean day is not the same as a blue moon. A constant source of annoyance is not typically associated with the idiom "once in a blue moon."