noun
two or three days; a couple of days
Used to express an approximate duration or a short period of a few days. Often appears in contexts like 二三日中に (within a few days) or 二三日かかる (it takes a couple of days).
二三日中にまた連絡します。
I'll contact you again within a couple of days.
この仕事は二三日かかると思います。
I think this work will take two or three days.
二日 means exactly 'two days', while 二三日 is an approximate range of 2–3 days.
Compound of 二 (two), 三 (three), and 日 (day). The reading にさんにち is a straightforward combination of the on'yomi for the numbers and the counter for days.