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

安全期[anzenki] safe period
暗黒期[ankokuki] eclipse period
一期[ichigo] one's life time
一期[ikki] one term
一期一会[ichigoichie] once-in-a-lifetime encounter (hence should be cherished as such)

一周期[isshuuki] period (in astronomy)
一定期間[itteikikan] fixed interval
延期[enki] postponement
延期になる[enkininaru] to be postponed
沖積期[chuusekiki] alluvial period

お花見[ohanami] cherry blossom viewing
一花[hitohana] success
卯の花[unohana] bean-curd lees
雲鬢花顔[unbinkagan] beautiful woman (metaphorical)
黄花[kouka] chrysanthemum

火花[hibana] spark
花の木[hananoki] flowering tree
花瓣[hanabira] (flower) petal
花より団子[hanayoridango] dumplings rather than flowers