What it means
中（じゅう） follows a noun that refers to a period or range of time and means “throughout” or “during the whole of” that period. It emphasizes that something continues or applies across the entire specified time, rather than happening at only one point in it. It is commonly used with expressions such as a day, a season, a vacation, or a year.
Usage note
Do not confuse 中（じゅう） with 中（ちゅう）. 中（じゅう） usually means “throughout the whole period,” while 中（ちゅう） often means “in the middle of” or “currently during” an activity or situation, as in 会議中（かいぎちゅう）.
How to use it
夏休み中、毎朝公園を走りました。
I ran in the park every morning throughout the summer vacation.
夏休み中、毎朝公園を走った。
I ran in the park every morning throughout the summer vacation.
Example sentences
夏休み中、私は毎朝図書館で宿題をしました。
Throughout summer vacation, I did my homework at the library every morning.
春休み中、学校の体育館は工事のため使えません。
Throughout spring break, the school's gymnasium cannot be used because of construction.
今年一年中、節電を心がけます。
I will make an effort to save electricity throughout this entire year.