The idiom ‘Once in a blue moon’ is used to describe an event that happens very rarely or almost never.

১. It originates from the astronomical phenomenon where a second full moon occurs in a single calendar month.
২. It cannot mean ‘always’ or ‘hourly’ as it emphasizes the scarcity of an occurrence.
৩. Example: ‘He visits his hometown once in a blue moon’.

অতিরিক্ত তথ্য: A blue moon occurs approximately once every 2.7 years.