Christmas beers are not really a traditional style but are more of a modern creation as breweries have looked to create seasonal products. That said, the Belgians have taken to it with gusto and have created some incredible brews. Very generally speaking, a Christmas beer is usually rich, strong and often heavy. It may also be spiced with herbs. And the alcohol is usually quite high (to warm you on the cold winter nights, of course).
To celebrate these creations each year, a regional Zythos beer tasters group throws the well regarded Kerstbierfestival. The 2-day event takes place in Essen, a town in the north of Belgium and a stone's throw from the border with Holland. This past year on December 12 & 13 they had on show 157 different Belgian Christmas beers to be sampled.
The event runs in this way: upon entry you get a sampling glass for €3 (refundable if you return it later) and then you buy tickets that you exchange for samples of the beers. There is a large board that lists the beers that are available, some on tap but most from a bottle. Volunteers do all the pouring. After deciding on what you want to sample, you head to the counter, use the glass-washing machine to rinse out your glass, tell them the number, and hand over a ticket.
As with other beer festivals I have been to in Belgium, this one had a bit of an older touch than you see in the U.S. The crowd was lively but quieter than ones I've been to back home. People would get their samples and then chat at their table over cheese. Many would take notes as they discussed their beers.
Onto the beers. I'd say that the Kessel Christmas and Winter 10 were my two favorite brews. They were both dark beers that really had that satisfying fullness that is so enjoyable on a winter night. Slaghmuylder's Kerstbier was very interesting in that it was an unfiltered lager and not a heavy dark beer as is most common. Still it worked because it had a nice strong malty base that was refreshing and sweet.