all summer; throughout the summer
Refers to the entire summer period. Often used adverbially with に or as a noun.
I spent the whole summer at the beach.
I want to finish reading this book during the summer.
Standard kanji spelling.
Mixed kana spelling; common in everyday writing.
Compound of 夏 (summer) and 中 (throughout, during). The reading なつじゅう follows standard rendaku for 中 when used as a suffix meaning 'throughout'.