夏季休暇[kakikyuuka] summer vacation
季刊[kikan] quarterly (e.g. magazine)
季語[kigo] seasonal word (in haiku)
季候[kikou] season
季節[kisetsu] season

季節はずれ[kisetsuhazure] unseasonable
季節的[kisetsuteki] seasonal
季節風[kisetsufuu] seasonal winds
四季[shiki] four seasons
冬季[touki] (season of) winter