一日中(いちにちじゅう)

Means "all day long" — from morning to night without interruption. Used as a time adverb, typically placed at the beginning of a sentence or before the verb.

PATTERN:
The suffix 中(じゅう) attached to a time period means "throughout that entire period."

SIMILAR EXPRESSIONS:

• 一年中(いちねんじゅう): all year round
• 一晩中(ひとばんじゅう): all night long
• 一日中(いちにちじゅう): all day long

NOTE ON READING:
Read as いちにちじゅう (not *いちにちちゅう). The suffix 中(じゅう) means "throughout" a time span, while 中(ちゅう) means "during" or "in the middle of" (e.g., 会議中(かいぎちゅう) = during a meeting).