Short answers, pulled from the story.
Winter occurs when the Earth tilts 23.44 degrees relative to its orbital plane and turns away from the Sun. This tilt causes sunlight to hit the surface at an oblique angle, reducing solar radiation per unit area.
The Julian calendar marked winter beginning on the 10th of November. Midwinter occurred on the 25th of December when the solstice fell within that system.
Gophers, frogs, snakes, bats, and bears enter a state of reduced metabolic activity known as hibernation. Snowshoe hares change their fur color to white to match surrounding snow for camouflage.
The Great Frost of 1683 froze the River Thames all the way up to London Bridge with ice reaching about two feet thick in London. The sea froze out around the coast of the southern North Sea causing severe shipping problems.
Freezing skin causes frostbite leading to loss of feeling and damaged tissue while trench foot results in numbness causing potential gangrene. Shivering leads to uncoordinated movements and can result in death from hypothermia.