If you need snow specifically for the festive season, choose the country first, then the altitude, and treat snowmaking as a fallback: resorts with mid-mountain at 2,600–3,000 m have never let you down at Christmas. This is the climate of 30 winters (1984/85–2013/14) plus model scenarios: climate, not a forecast, and not today's report. For current snow, check the resort's snow reports.
The country matters more than the name
The site counts a resort as snow-sure if it has a snowy Christmas in 25 or more of 30 winters. In Switzerland that's 63 of 95 (median 28 of 30), in Austria 62 of 134, in France 30 of 125, in Italy 19 of 117. Up north, Sweden delivers 26 of 34, Norway 23 of 29. In Czechia and Poland, not one: Christmas there depends on snowmaking.
Altitude versus snowmaking
Mölltaler Gletscher (2,600 m) and Passo Stelvio (3,000 m) delivered snow on 30 of 30 Christmases. Schia Monte Caio (1,300 m) and Glencoe (700 m) saw no natural snow at all; in the models, snowmaking lifts them only to 9 and 17 of 30.