Usually, we experience a full moon once a month because the monthly calendar was built on the lunar cycle (29.5 days). But once every 2-1/2 years, those extra half days add up to two full moons in a month.
In 1943, Sky and Telescope Magazine erroneously wrote that the second full moon in any calendar month was called a blue moon. The label stuck and is still used today. It’s relatively rare that a blue moon would fall on New Year’s Eve. The last time that happened was 1990. Even more rare – there were two blue moons in 1999, one in January and one in March. That happens only about four times a century.
Interestingly enough, this is not where the expression “once in a blue moon” comes from. According to NASA, that phrase is believed to have originated in 1883 after the eruption of Indonesia’s Mount Krakatoa. The volcano put so much dust in the atmosphere that the moon actually looked blue in color. The event was deemed so unusual the phrase “once in a blue moon” was coined.
Take a moment to go to the blog of my very talented friend Julie at Being Ruby for her gorgeous photographs of the Australian Blue Moon - one of which I swiped - and you'll see why I feature her blog.