Historically, Germanic peoples thought of the year in terms of two seasons rather than our modern four (much like their neighbors, the Celts). These seasons were summer and winter, and were roughly divided between long-days and short-days. Therefore, one of the most important days of all was the beginning of winter, also known as Winter Day. The Ynglinga Saga, written by Icelander Snorri Sturluson around 1225 (wikipedia is helpful for remembering specifics), mentions three sacrifices, including this one: "On winter day there should be blood-sacrifice for a good year," (Ynglinga Saga, chapter 8). Modern Germanic Pagans such as the Asatru have adopted this holiday, though in the Heathen community it becomes more of a harvest and ancestor celebration; actual blood sacrifice being relatively rare these days (Asatru holidays). The Catholic holiday of St. Martin's Day is thought to be a remnant of this old celebration (Albertsson, Travels Through Middle Earth). For many modern Heathens, the celebration is held in early or mid-November; for others, it is held in mid-October - the variance of seasons across the Germanic lands makes pinning down a specific date very difficult.
The Anglo-Saxons, a Germanic people that made England their home sometime in the fifth century, are thought to have held their celebrations near the Full Moon in the month (look here for more awesome AS calendar info), therefor I have taken to celebrating Winternights near the Full Moon of Blótmónað (roughly corresponding to our month of November). So on Sunday night, my children and I will be cooking a wonderful feast, pouring out some ale to the deities, ancestors, and land wights, and talking about the change in season that has taken over the land. Winter nights are here.
Just as a note: my Germanic path is rather loose Anglo-Saxon Heathenism, and I am not as familiar with the Old Norse sources as some Asatru or continental German reconstructionists. So if I have anything incorrect or in a misconstrued context, please let me know in the comments - I'm always open to correction!