seasonal

adj. pertaining to the seasons of the year; periodical, occurring only during particular times or seasons

Seasonality of food refers to the times of year when a given type food is at its peak, either in terms of harvest or its flavour. This is usually the time when the item is the cheapest and the freshest on the market. The food's peak time in terms of harvest usually co-incides with when its flavour is at its best. There are some exceptions; an example being sweet potatoes which are best eaten quite a while after harvest. This page serves as a guide to the seasonality of food. The list below is for foods common in the Northern Hemisphere.
See more at Wikipedia.org...

Adjective
1. occurring at or dependent on a particular season; "seasonal labor"; "a seasonal rise in unemployment"
(antonym) year-round, year-around

(a.)
Of or pertaining to the seasons.
  

Allergic rhinitis (hayfever) which occurs during a specific season.