Celtic and Jewish calendars each have 13 months. Interincluded, together they have 26 months (where 26 is the gemaria of the 4-letter Divine Name, the shem ha-etzem). The 13th month of the Celtic calendar is Ruis; of the Jewish calendar, the 13th month is Adar 2, the month of atzilutic interinclusion. Ruis occurs during winter (while Adar 2 occurs as winter turns to spring), the season of terrible ice (malchut of atzilut in a congealed state, of undefined awareness) and snow (chochmah in a state of permanence, memory). Ruis includes the winter solstice and culminates on a day/night set aside apart from any month. This day/night is called the unhewn dolmen, an unhewn stone marking the sacred doorway to the mysteries. The Celtic calendar operates on a 19-year cycle, as does the Jewish calendar; both intercalate solar time into the basically lunar calendar.
Ruis is the month of the blue moon and of mystic completeness; of that called tamim/temimut in Hebrew. A blue moon, defined as two full moons within a single civil (Gregorian) calendar month, occurs 7 times during a 19-year metonic cycle of lunar phases. A blue moon is associated with rarity, and a mathematical probability of 8 in 228, or 3.5%.
In the Jewish calendar, as opposed to the civil calendar, there can never be a blue moon month. The full moon is always mid-month, although this may not necessarily coincide with the astronomical full moon.
In the Celtic determination of time, blue moons can be. In the Jewish determination of time, blue moons can be not. Consequently, between the two calendars, reality can dance between being and not being, between something and nothing. Koach mah? What is the genius in this?
The ordinal value of Chavah (Eve) is 19. The 19th letter of the alef-beit is kuf, the first letter of kadosh, holy. The first 19 physical, the second 19 spiritual, are both bound together in unity using two systems of 19. Thus, the physical and the spiritual are unified through the essence of both Am Yisrael and the nations.
Double blue moons, 2 months of a blue moon in 1 year, are even rarer. The year of my birth,1961, was a year of double blue moons, a double portion of that which occurs 7 times in a 19-year cycle. The doubling of 7 is 14, the number of hours that the true full moon may differ from the calculated one.
Hashem desired a dwelling ... Midrash Tanchuma Naso 16