Lunar months are (approximately) 29 1/2 days, so twelve of them make 354 days. The people who built Stonehenge obviously included careful scholars of the apparent movement of celestial bodies. With a very ignorant and superstitious population, getting the days of the solstices and the equinoxes was very important, and gave priests a great deal of power. But the Druids were not priests of that variety, and the Kelts were sufficiently intelligent not to need anyone to tell them that "the sun will come back" in the springtime. Samhain and Beltane (roughly, May 1st) were festivals when the already lax social code was almost abandoned. Sexual license was popular, and a good many children were the product of those festivals. Keltic women went around armed, and were often expert in the use of weapons. Women past child-bearing age, or who did not intend to bear children usually instructed the children in the use of weapons. Setanta/Cu Chulainn (it means the Hound of Cullen), after he became the Hound, went off to what we call Scotland, where he was trained in the use of weapons by two women there. Women exercised a good deal of power in Keltic societies, and the great mythic cycle of the Irish, the Táin Bó Cúailnge (Cattle raid of Cooley) arose because Medb and her husband Ailil of Connacht, got into a dispute because the head of a household was the man or woman with the most wealth. Medb and Ailil had equal wealth, but for Madb's great stud bull, the White Bull of Leinster. The bull, however, was shamed by the thought that he would cause Ailil to lose status, so in the night, he stole off to join Ailil's herd. Madb then convinced Fergus, an exile from Ulster, to lead a cattle raid to capture Cooley's stud bull, the Brown Bull of Ulster. This would restore the balance, and Madb at least would not be dominated by her husband. With a curse om the men of Ulster, Cu Chulainn, the Hound of Cullen, was the only defense of Ulster--he'd been in Scotland when the curse was called down on Ulster. Although still just a teenager, Cu Chulain held off the men of Connacht until the curse on the men of Ulster wore off.
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Queen Madb, and Cu Chulainn defending the ford.
What has all this to do with Samhain and Beltane? Many children were festival-got, and with women well-armed and skillful in the use of weapons, you didn't want to question them too much about who the father or fathers of her children were. In fact, the woman might not know. For this reason, men didn't know if a male child was their own son, so men lavished their attention and care on the on the sister's son--they knew he had the same blood as they did. Samhain was the big festival, before winter closed in. Nekkid dancing and slipping off into the shadows were simple and popular entertainments.
Christians are a dour, humorless and forbidding lot. The ancient Kelts, before they were infected with the disease of christianity, had a lot more fun, and a much richer culture and more egalitarian society. Christians screw everything up.