The Old English Lunar Calendar

The Old English lunar calendar gave names to the full moons of the year based upon the season in which they fell, e.g. the first full moon after the winter solstice is the Moon After Yule, and the first one following the spring equinox is the Egg Moon. Originally, this may have been the basis for a solilunar calendar with the old saxon month names, but there is some conjecture over how this calendar would have worked. The names used for this calendar are more recent, but there are many different sources with different lists of names for the full moons, so for this calendar I have used some common names that have been used in the past. The list I have chosen pays cognisance to the fact that many of the names of the full moons are related to the solar month in which they fall, in this case according to the Old English solar calendar. Many of these names appear in this Wikipedia article.

Full Moon Name Date Range
Moon After Yule Dec - Jan
Snow Moon Jan - Feb
Lenten Moon Feb - Mar
Egg Moon Mar - Apr
Milk Moon Apr - May
Flower Moon May - Jun
Hay Moon Jun - Jul
Grain Moon Jul - Aug
Fruit Moon Aug - Sep
Harvest Moon Sep - Oct
Hunter's Moon Oct - Nov
Moon Before Yule Nov - Dec

To keep the moons in line with the seasons, however, an "extra" moon has to be inserted every now and then. Each season normally has three full moons, but when a season has four full moons, the third one is designated as the "extra" moon. This ensures that certain full moons, such as the Moon Before Yule and the Egg Moon, which must occur at a certain time in the season, are not displaced. This extra moon is called the Blue Moon. This article in Sky & Telescope claims that this rule is the correct and traditional method of defining a blue moon, and it is the method adopted for this calendar.