How can we find out where there will be snow in Europe in December?

The answer is going to depend on the country. There is no worldwide answer.

National meteorological institutes often have maps of snow frequency. Not that you want snowcover, which may be harder to find than snowfall. Some places get lots of snowfall that melts within days, whereas others might get only little snowfall, but are so cold that any snow falling October or later is sure to remain until April at least. Interpreting precipitation + temperature statistics to estimate snowcover frequency is possible but error-prone. The answers, per country, below, are all based on snowcover data.

Sweden

For example, this map shows the probability of a "white christmas" (at least 5 cm of snow on 24, 25, 26 December) in Sweden:

^{Frequency of White Christmas, 1931-1980. Source: SMHI}

You can safely extrapolate from the gradient in this map that anywhere further south, you can't be sure to see snow (except in the mountains).

Norway

Similarly, you can browse senorge.no to get historical snow observations, although I don't know if they have a frequency map like the Swedish do.

Finland

Finnish FMI has a set of maps with snow statistics, as well as White Christmas probabilities:

^{Frequency of White Christmas, Source: FMI}

Canada

I don't know about maps for Canada. I'm quite sure that on a map covering all of Canada, all will have close to 100% probability of snowcover in winter. Results for deep snow (say >30 cm) might be more interesting. However, Canadian Climate Normals can give you snow frequency per month for any weather station. For example, in January, Toronto has on average 21 days with >1 cm snow per year, Winnipeg has 31, Victoria only has 3.2, and St. Johns has 27.

In an answer to a previous question about weather, there was a reference to this website called Weather Spark that turned out very great for historical values.

There is no relevant graph beside maybe the probability of precipitation (but it does not say how thick the snow will be) and the snow depth (but there does not seem to be any data). Graphs are accessible through the "Dashboard" tab.

However for each destination there is a yearly report on many topics ("Averages" tab), including snow. I took the example of Annecy, in the French Alps, close to the mountains, and the highest probability of snow falls is between December and January, with over 20%. So for this destination, it would fit your schedule.

I think it is possible to play with this website to find other tools more appropriate to evaluate the snow height (I like the graphs as well but it is harder to evaluate what probability is high enough [the website apparently evaluates the "snow season" when it is above 8%]).