How did the idiom ‘once in a blue moon’ originate ?

The idiom ‘once in a blue moon’ means something that happens very rarely.

Examples : “My grandparents used to go for a movie once in a blue moon” * “My husband gives me a gift once in a blue moon”. 

There are many theories as to the reference to ‘blue moon’ in this idiomatic expression. One of the theories is that ‘blue moon’ refers to the moon which turns blue due to the dust particles in the atmosphere after an exceptionally violent volcanic eruption.  In places where such eruption occurs the moon appears bluish at night due to the presence of extremely high dust particles in the atmosphere. This is something that happens very rarely hence the saying ‘once in a blue moon’.

According to another version ‘blue moon’ refers to second full moon in a single solar calendar month. This does not happen every month but only once in two to three years. The uncommonness of this phenomenon led to the origin of the term ‘blue moon’.