The short answer
Almost never in the city itself. What you do get, from roughly November to April, is a clear view of snow on the High Atlas peaks behind Marrakech. Real snow is an hour or two away in the mountains, and the Oukaïmeden ski station, about two hours from the city, opens in a good winter.
In more detail
Marrakech stays mild in winter, with highs of about 19 °C in January and lows of about 6 °C. Snow in the city is extremely rare. The view south, though, is snow-capped for much of the cold season: the High Atlas rising behind the Koutoubia is the classic winter picture.
If you want to stand in snow, go up. Imlil, the trailhead for Toubkal, and the upper Ourika valley see snow in winter; our New Year notes call snow on Toubkal typical in January. Oukaïmeden lets you ski in sight of the plain and be back in Marrakech for dinner. Closed shoes with grip and a warm layer are essential.
Snow also affects roads. The Tizi n'Tichka pass on the way to Ouarzazate and the Sahara can close after a heavy fall, most often in January and early February. Mountain day trips go ahead in most winter weather, but check conditions the day before.