No: by this site's standard (a snowy Christmas in at least 25 winters out of 30), Slovenia does not have a single snow-sure resort out of its 7. The median managed just 12 snowy Christmases out of 30 over 1984/85–2013/14. Closest to the threshold are Krvavec above Ljubljana and Sella Nevea - Kanin in the Julian Alps: both sit in the 1,700 m height band and both scored 18 out of 30.
Holiday skiing here runs on snow cannons
Lower down there is very little snow at Christmas. At Kranjska Gora and Vogel at 1,200 m there was natural snow over the holidays in 8 winters out of 30; at Smučišča Mariborsko Pohorje at 800 m, in 3. With typical modelled snowmaking, Kranjska Gora reaches 30 out of 30, and all seven resorts clear the threshold: holiday skiing here lives on artificial snow. These figures describe the climate of past winters; for fresh snow, go to the resorts' own snow reports (methodology). If you need snow specifically at Christmas, Austria is the better bet, with 62 snow-sure resorts out of 134; Italy has 19 out of 117. For Slovenia, come between January and March.