noun
23rd day of the month
Used for dates; the 23rd day of any month.
会議は二十三日に延期されました。
The meeting was postponed to the 23rd.
noun
23 days
Used for a duration of 23 days. Less common than the date sense.
このプロジェクトは二十三日で完了した。
This project was completed in 23 days.
Means 'two or three days', not '23rd' or '23 days'. The kanji 二三日 can be read either way, but にさんにち is far more common.
Compound of 二十 (にじゅう, 'twenty'), 三 (さん, 'three'), and 日 (にち, 'day').