all day and night; a whole day and night
Refers to the entire period of one day and one night, often used in contexts emphasizing continuous duration or a complete 24-hour cycle.
We arrived at our destination after traveling all day and night.
Compound of 一日 (いちにち, 'one day') and 一夜 (いちや, 'one night'). The reading is a straightforward combination of the readings of its parts.