The Collaborative International Dictionary of English v.0.48:

Seasonal \Sea"son*al\, a.
   1. Of or pertaining to the seasons.
      [1913 Webster]

   2. Occurring or being used in a specific season; as, seasonal
      items for sale.
      [1913 Webster]

   Seasonal dimorphism (Zool.), the condition of having two
      distinct varieties which appear at different seasons, as
      certain species of butterflies in which the spring brood
      differs from the summer or autumnal brood.
      [1913 Webster]