The concept behind the origination of this idiom is second but rare appearance of the full moon in same month. Blue moon is the full moon that appears second time in same calendar month and this phenomenon happens only once in 32 months. Apart from that, sometime the full moon appears to be different in color especially blue and orange and bigger in size. This idiomatic expression is in use since 1800s.
[ Examples ]
1. I don't know why she bought that music system. She uses it once in a blue moon. 2. I think my grandson doesn't love me anymore, he comes to see me only once in a blue moon. 3. Although I trust in God, I visit that famous temple only once in a blue moon.
