Everybody knows the phrase - 'Once in a blue moon' means very rarely. Not never, like 'When two Sundays come together', or 'Когда рак на горе свистнет' - no, just very rarely. And today is just the day! Astronomically it means an extra full moon in a season - 4 instead of 3; or an extra full moon in a year - 13 instead of 12; or two full moons in a month. It has nothing to do with its colour.
Well, this year is quite unusual. It does have 13 full moons rather than 12. But also, there were two full moons in January (1st and 31st), no full moons at all in February, and two full moons again in March (1st and 31st), one of which - the Blue one - is today! A good time to sing a song:
My name is Elena Rafaelevna Watson, I have been teaching English as a foreign language for over 25 years now. I have also been translating and interpreting (English/Russian) for over 20 years.