Short answers, pulled from the story.
Winter is caused by the 23.44-degree tilt of Earth's axis relative to its orbital plane. When a hemisphere tilts away from the Sun, sunlight strikes the surface at a more oblique angle and travels a longer distance through the atmosphere, reducing the amount of solar energy reaching the ground. The effect of the Earth's elliptical orbit on its distance from the Sun is negligible compared to axial tilt.
There is no single start date for winter. Meteorologists count December, January, and February in the Northern Hemisphere. Celtic traditions began winter on the 1st of November. The Julian calendar started it on the 10th of November. Germanic regions marked its start on the 11th of November with Martinmas. Astronomers place its start at the winter solstice in late December. Chinese astronomy begins winter on or around the 7th of November on Lidong.
The winter solstice is the moment when the Sun's elevation relative to the North or South Pole reaches its most negative value - the Sun is at its farthest below the horizon as seen from the pole. That day has the fewest hours of daylight of the year. The coldest temperatures typically arrive weeks after the solstice due to seasonal lag.
Animals use four main strategies: migration, hibernation, food storage, and physical resistance. Hibernators such as gophers, frogs, snakes, bats, and bears enter a state of reduced metabolic activity. Animals like squirrels, beavers, and raccoons store food rather than hibernating fully. Species including the Arctic fox, rock ptarmigan, and mountain hare change their coloring to white for camouflage in snow.
The Year Without a Summer was 1816, caused primarily by the eruption of Mount Tambora in Indonesia in April 1815. The volcanic effects produced an unusually cold winter in 1815-1816 and prevented summer from recovering in the Northern Hemisphere. Frost killed crops in New England in May, snow fell in New York and Maine in June, and ice formed on lakes and rivers in July and August, resulting in the worst famine of the 19th century.
Winter appears in mythology as the consequence of grief, divine conflict, or seasonal battle. In Greek myth, Demeter's depression over Persephone's captivity with Hades causes winter for six months each year. In Persian culture, the winter solstice is called Yalda, meaning "birth", and marks the eve of the birth of Mithra, who symbolized light and goodness. In Welsh mythology, winter is represented by the ongoing battle between Gwyn ap Nudd and Gwythr ap Greidawl.