What are the best Fall/Winter soups that can be made the day before serving to a crowd?

Pasta fagiole, bread soup, goulasch soup. black bean soup, lentil or split pea soup--lots of choices!

I agree with la domestique and butternut squash soup is excellent too the next day. You could also do the recipe for lentil and sausage soup for a cold winter's night on this site (cannot link on this device) it is a full healthy meal on its own and you don't have to serve anything else except maybe some cheese and olives as an appetizer. I love it with kale instead of spinach because of the extra crunch. Make the soup but don't add the greens the night before - just have them washed and prepped in a plastic bag. When you ready to serve, bring the soup up to a simmer and add the greens - they only take a minute or two to cook. The recipe serves a lot of people but you could easily double it if you have a big pot, or thin it out slightly by adding more stock

Pureed vegetable soups are great for making a day ahead. I've got a recipe for Pumpkin Soup that's flavored with apples, sage, and hazelnuts:
http://www.ladomestique.com/2012/10/22/cook-in-the-moment-pumpkin-soup/