Usually in summer. Hail is caused by the freezing of rain, which is the condensing of moisture caused by temperature differentials. Hail is usually formed high in the atmosphere where rising hotter air is slowing and condensing because of cooler upper air. This quick change in pressure causes the particles to collect and if cool enough, freeze into chunks as it falls. As these chunks fall, they also start to create friction(heat) with the air and slowly melt (hopefully to a size that will not cause too much damage by the time it reaches the surface). Snow is caused in much the same way and also need a fair amount of heat to begin. When the temperatures are lower, the amount of condensation is less and the energy required to raise the moisture higher in the atmosphere is less therefore crystals are formed rather than larger pieces like rain or hail. There are ice storms in the winter but I believe that refers to the rapid melting and refreezing of snow and ice and other moisture at ground level and does not precipitate out of the atmosphere.
