nth day (of the month)
counter for days
「日」 has other readings with different meanings.
day; days
Sunday
day of month
荷物は三日のうちに届きますよ。
It'll arrive on the third.
何日のうちが都合いいですか？
What date works for you?
二十日んちまでに返してね。
Return it by the 20th.