This is the 3219th most frequent Japanese word.
夏季
Summer season.
Used as a noun to indicate the season of summer.
夏季は日本で花火大会が多く行われる季節です。
The summer season is when many fireworks festivals are held in Japan.
Used as an adjective to describe a vacation period occurring in summer.
学校の授業は夏季休暇に入ります。
The school's classes enter the summer vacation period.
Used as an adjective to detail items intended for summer use.