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).
I'll contact you again within a couple of days.
I think this work will take two or three days.
I'll send my reply within a couple of days.
I was told the repairs would take a couple of days.
He called me a couple of days ago.
Please come again in a couple of days.
二日 means exactly 'two days', whereas 二三日 is an approximate range of two to three days, and learners can mistake one for the other due to the shared kanji.
二十三日 (にじゅうさんにち) means the 23rd day of a month or a duration of 23 days, while 二三日 (にさんにち) means a vague 'two or three days' even though it can share the same kanji spelling.
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.