The winter solstice — the shortest day and longest night of the year in the northern hemisphere — falls around 21–22 December. The sun rises at its most southerly point, reaches its lowest maximum height and sets at its most southerly point. The word "solstice" means "sun standstill" — the sun appears to pause before reversing its course, rising a little further north each subsequent day until the summer solstice six months later.
The winter solstice has been marked by human beings longer than any other astronomical event. Newgrange in Ireland — a passage tomb built around 3200 BCE, over five thousand years ago — is precisely aligned to the midwinter sunrise: on the mornings around the solstice, the rising sun penetrates the entrance passage and illuminates the inner chamber for approximately 17 minutes. The builders of Newgrange watched the solstice sunrise for generations before they built a monument to capture it — and then they built one of the most perfectly engineered structures of the ancient world to do so.
The name "Yule" comes from the Old Norse jól — the great midwinter feast of the Norse world, lasting twelve nights. It was a time of feasting, the sacrifice of animals for the winter feast, the drinking of toasts to the gods, and the wild hunt of Odin across the winter sky. The twelve days of Christmas and the figure of the gift-bringing old man (Odin, who was said to bring gifts to those who left offerings for his horse Sleipnir) are among Yule's most direct survivals in modern Christmas tradition.
The Roman festival of Saturnalia — the great winter festival of Saturn, god of agriculture and time — fell in late December and involved gift-giving, feasting, role reversals (masters served their slaves) and general suspension of ordinary social norms. The Roman Dies Natalis Solis Invicti ("Birthday of the Unconquered Sun") was celebrated on 25 December — the date that the Roman calendar placed the winter solstice and that the Christian church eventually adopted for Christmas.