Summer vacation; time off taken during the summer period (often called 'natsu-yasumi' in everyday speech).
今年の夏季休暇は家族で沖縄へ行く予定です。
This year's summer vacation we plan to go to Okinawa with the family.
会社の夏季休暇は八月の第2週に設定されている。
The company's summer holiday is set for the second week of August.
学校は長い夏季休暇の間に補習を行うことがある。
Schools sometimes hold remedial classes during the long summer vacation.