A Blue Moon is commonly the name given to the second full moon in a calendar month. Since a full moon occurs every 29 1/2 days, if there is a full moon on the 1st or 2nd day of a month, there is a good chance there will be a second full or blue moon that month. Blue moons happen every two to three years. The moon looks like a regular full moon and not blue in color.
'Once is a blue moon' is frequently used expression which means ‘not very often,’ or ‘very rarely'.
'Once is a blue moon' is frequently used expression which means ‘not very often,’ or ‘very rarely'.