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nth day (of the month)
counter for days
「日」 has other readings with different meanings.
day; days
Sunday
day of month
荷物にもつは三みっ日かのうちに届きとどきますよ。
It'll arrive on the third.
何なん日にちのうちが都合つごういいですか？
What date works for you?
二十はつ日かんちまでに返しかえしてね。
Return it by the 20th.