Once in a Blue Moon.
Meaning something which doesn't happen very often.
So, what is a Blue moon?.
In most cultures throughout time the full moon was given a name that Symbolises the time of year ie. Harvest moon in Autumn.
There are twelve names for the full moons, but as the full moon cycles every 29.5 days this means that every 2 to 3 years there are thirteen full moons in a year.
This thirteenth full moon is called the 'Blue Moon'
When do Blue Moons occur?
This is where it starts to get a little complicated, this all depends on how we allocate names to the full moons, the modern method is to name the moon according to which calendar month the full moon occurs.
Using this method, a blue moon will be the second full moon in any calendar month, however this may differ for different parts of the world (due to different time zones).
The traditional naming scheme was to give the full moon names according to the season based on the times of solstice and equinox, so normally each season has three full moons, but again once every 2-3 years one season will have four full moons. In this case the third of the four full moons will be a Blue Moon.
But it doesn't stop there, in the ecclesiastical calendar the dates for the movable feasts such as Easter and lent are governed by the spring equinox and the first full moon after March 21st (fixed date for spring equinox).
The naming of the moons start with the first full moon after March 21st (Paschal moon) and are named consecutively until the last one (Lenten moon) the moon just before march 21st the following year.
Because we get an extra full moon every 2-3 years this causes the Lenten moon to appear too early, IE not the moon before the 21 March, so a intercalary (blue) moon is inserted so that the Lenten moon is once again in the correct place. *see below*
2019 Has one seasonal Blue Moon on 18 May, this is the third of four full moon's in spring, this is considered by some as a 'True' Blue Moon according to the traditional definition.
Northern Hemisphere full moon names by month:
January: Old Moon, Moon After Yule
February: Snow Moon, Hunger Moon, Wolf Moon
March: Sap Moon, Crow Moon, Lenten Moon
April: Grass Moon, Egg Moon, Pink Moon
May: Flower Moon, Planting Moon, Milk Moon
June: Rose Moon, Flower Moon, Strawberry Moon
July: Thunder Moon, Hay Moon
August: Green Corn Moon, Grain Moon
September: Fruit Moon, Harvest Moon
October: Harvest Moon, Hunters Moon
November: Hunters Moon, Frosty Moon, or Beaver Moon
December: Cold Moon, Moon Before Yule, or Long Night Moon
Full moon names by season (Northern or Southern Hemisphere):
After the winter solstice:
OLD MOON, or Moon After Yule
Snow Moon, Hunger Moon, or WOLF MOON
Sap Moon, Crow Moon or LENTEN MOON
After the spring equinox:
Grass Moon, EGG MOON or Paschal Moon
Planting Moon, or MILK MOON
Rose Moon, FLOWER MOON, or Strawberry Moon
After the summer solstice:
Thunder Moon, or HAY MOON
Green Corn Moon, or GRAIN MOON
FRUIT MOON, or Harvest Moon
After the autumnal equinox:
HARVEST MOON, or Hunters Moon
HUNTERS MOON, Frosty Moon, or Beaver Moon
Moon Before Yule, or Long Night Moon, OAK MOON
* This is a simplification as the ecclesiastical lunar calendar follows an alternating 29/30 day lunar cycle and does not always coincide with the astronomical full moon. see http://en.wikipedia.org/wiki/Computus for more details