The Christmas beers have a stronghold in Norway. Given the current state of affairs, that is kind of surprising. They are released the first week of November every year. There are two kinds, one at ~4.7% (class D) and one at ~6.5% (class F). It is the strongest one that is the traditional one, but it lost most of its market when strong beers where moved to Vinmonopolet, the government owned stores, about a decade ago.
They are all lagers, which I believe is a direct consequence of all the Norwegian breweries being lager breweries. Recently microbreweries have started making Christmas beers too, but they do not typically fall into this beer style.
If you've never tried one then expect something in-between a Münchener dunkel and a Doppelbock.
Norbrygg, the Norwegian Homebrewers Association, defines the style as follows:
Alcohol by volume:
Low to medium hop aroma, and a small fruity note. Malty rich aroma.
The colour spans from copper to dark brown.
The traditional Christmas beer is a lager with a prominent malt flavour and a medium bitterness and next to none hop flavour. A little fruitiness may be found.
Medium- to full-bodied.
Aass juleøl, Ringnes juleøl.
For those of you who understand Norwegian I recommend this article.