@dt426 For Sims 3, the festival grounds have nothing to do with the weather (as opposed to the seasons), so you can set that part aside. Having snow on the ground is dependent on temperatures as well as whether it's snowed recently.
You can choose your weather and snow cover by opening the cheats console (ctrl-shift-C) and entering testingcheatsenabled true , then shift-clicking the ground and selecting Seasons. Please do this and see whether you can force it to snow, and keep snowing, and create a layer of snow on the ground.
