The calendar of the Celtic year is very different than the one that we use now. It had 13 months, twelve of them had approximately 30 days and the last had only three. The months were divided into two fifteen-day periods, one was the waxing phase (or the light phase) and one was the waning phase (dark phase) of the moon.
The year was also divided into a dark and light half, with the year starting at the dark half on Samhain and the light half beginning at Beltainne. The Celts considered the day to begin with night, and the year also began with the dark half first.
The graphic above shows the Celtic calendar as they perceived it, a giant wheel with the four great festivals, Samhain, Imbolg, Beltainne and Lughnassadh, as the spokes. I have also labeled the soltsices and equinoxes because, although no record exists showing the ancient druids to acknowledge these, modern-day druids count them as great holidays too.
On the illustration, the yellow lines represent our modern months and the unlabeled green lines in the center show the Druidic months as shown on the Coligny Calendar. I have listed them for you below, with the year starting on Samhain, that is, Nov 1.
Ruis which only has three days
As you can see, the year was quartered by the four major Celtic holidays, called the fire-festivals on account of the massive bonfies lit at these times. Below is a short explanation of the signifigance of each.
Samhain (SOW-an)- This day was the end of the old year and the beginning of the new. Between times like these, those times not part of one side or the other, were considered sacred bacause the veil sheilding the Otherworld was lifted and beings might pass to and fro more easily. This was also celebrating, or perhaps mourning, the coming of winter and the dark half of the year.
Imbolg (IM-bulc)- This holdiay celebrated the coming of spring and the lifting of the frost, or more specifically, the ewes coming into milk for their lambs. It was a holiday sacred to Brighid and all offerings this day were made to her.
Beltainne (bell-TAWN-a)- This very popular coming of summer festival is still celebrated throughout the world as May-Day, complete with maypoles and frolicking maidens. It was sacred to the Gaelic fire and light god Bel, about whom not much else is known. Beltainne symbolised the mating of the mother earth with the horned god of the hunt and therefore, mating among mere mortals was also encouraged on this day.
Lughnassadh- (loo-NAH-sah)-This, however sensible it may seem, did not celebrate the coming of autumn because the Celtic year did not include autumn as a season, it was simply part of summer. Instead this was a harvest festival and a chance for friendly competition between neighbors as the primary event of this holiday was the games. The games were supposed to represent the games the god Lugh (The major deity of this holiday) held in honor of his foster mother when she died.
The Celts also recognized the Great Year, a period of nineteen years when both the sun and the moon return to the exact same position in the sky. Some believe this may be the reason that Druidic training was supposed to have taken nineteen years to complete.