Technically, the snow doesn't really evaporate. Only liquids can evaporate in the scientific sense of the word. To truly evaporate, snow has to first melt into ice, then become water vapor. A liquid evaporates when there is enough energy in the molecule for the liquid to escape into the air. Once it escapes, it becomes water vapor. The energy that causes evaporation usually comes from sunlight or heat.
Sublimation of snow works exactly the same way as evaporation, except melting doesn't occur. The molecules of the ice are excited with energy, and the ice molecules go directly from a solid to a vapor. If you've ever seen dry ice "evaporate" into wisps of fog, you are actually seeing sublimation. The frozen carbon dioxide in dry ice is turning into carbon dioxide gas. Frozen water takes a lot longer to sublimate than dry ice.
A major factor in the evaporation of water is heat. But with sublimation, sunlight plays a more important role. That is why snow is often deeper under trees and under shaded areas compared to areas that get a lot of sunlight. The energy in sunlight makes the molecules move faster so they're more likely to break off into the air. Relative humidity also plays a part. Sublimation typically takes place where there are dry winds. If the air is dry, it is more likely to absorb molecules from ice. Low air pressure also promotes sublimation.
Sublimation is more likely to occur in late winter as opposed to early winter because the sun is more directly overhead. The energy from light photons is more intense because the sun is shining almost straight down in late winter, transferring more energy to the ice molecules. In early winter, the sunlight comes down at more of an angle, transferring less energy to the ice.