Until recently, it was widely believed that ice was slippery because the pressure of an object in contact with it caused a thin layer to melt. For example, the blade of an ice skate, exerting pressure on the ice, melted a thin layer, providing lubrication between the ice and the blade.
This explanation is no longer widely accepted. There is still debate about why ice is slippery. The explanation gaining acceptance is that ice molecules in contact with air cannot properly bond with the molecules of the mass of ice beneath (and thus are free to move like molecules of liquid water). These molecules remain in a semiliquid state, providing lubrication regardless of pressure against the ice exerted by any object. 
This phenomenon doesn't seem to hold true at all temperatures. Extreme conditions, found especially in Antarctica, have been observed to make ice and snow lose their slippery qualities. Explorers report that, at very low temperatures, snow loses its "glide", and pulling a sledge across it becomes like pulling a sledge through sand.
Hey all that time of year is around the corner and thought I'd give you some "brain food" Stay safe.