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16th day of the month
16 days
「十六日」 has another reading with different meanings.
十六日に届いた手紙にすぐ返事を書いた。
I immediately wrote a reply to the letter that arrived on the 16th.
来月の十六日から新しい制度が始まる。
The new system starts from the 16th of next month.
十六日は友人の誕生日パーティーがある。
There's a friend's birthday party on the 16th.