Both the Greeks and the Celts considered the number nineteen
greatly significant and this played a part in the calculation of
the year cycles. The period of nineteen years, the Golden Year,
is referred to frequently and is represented, for our purposes,
in it's astronomical context. It is the time period for the apparent movement of the sun and stars and the sequence of lunation's to coincide. That is the period of time taken for the lunar and solar calendars meet before diverging again. The exact period is 18.61
years, or 6,797 days. The Golden Year was divided into nineteen
year units, each unit was divided into four sections governed by
the midsummer and midwinter solstice and the vernal and
autumnal equinox. These all have a slightly variable date
because of the earth's eccentric orbit and the angle of 10 degrees between the line of the solstices and the major axis of the earth's ellipse giving rise to different time periods between each transit.
For convenience, the dates are usually given as 21^st March, vernal
equinox; 21^st June, summer solstice; 21^st September, vernal
equinox and 21^st December for the winter solstice. For example,
the spring equinox may fall on either the 20^th or 21^st May
depending when the alignment takes place.

There is a large amount of inferential evidence to be drawn
from Celtic history, art and mythology, as well as classical and vernacular sources, which suggest the Celts maintained effective astronomical observations and a sophisticated esoteric calendar,
both of the year and the greater cycle of the Golden Year. This
was written down, having been passed from generation to
generation orally in the Middle Ages in variously bowdlerized
forms in legends and mystery tales. There was also the Coligny Calendar, but that is incomplete, remaining in fragmented form.

The year was further subdivided into a total of eight section
governed by the moon's cycle. These dates were later
incorporated into the church calendar as Christian Holy Days.
I have covered the chain of events leading to these adoptions in
the individual festival's information page.

Samhain 31^st Oct - 2^nd Nov, became All Saints and All Hallows

Alban Arthuan or Midwinter 21^st Dec, became Christmas

Imbolic 2^nd Feb, became Candlemas

Alban Eiler 21^st March, became the Easter period

Beltain 1^st May, became May Day (celebrated but not Christian)

Alban Heruin 21^st June

Lughnasdh 1^st Aug, became Lammas

Alban Elued 21^st Sept became Harvest Festival

Each of the Celtic festivals and their subsequent month
corresponded with a comparative 'age of man', each also had
it's own plant and, in more recent times, letter associated with it.