The 15th day of the month (a calendar date). Usually read as 'jūgo-nichi'.
今月の１５日は祝日ではありません。
The 15th of this month is not a holiday.
請求書の支払いは１５日締めです。
Invoices are closed on the 15th for payment.
ミーティングは１５日の午後に行います。
The meeting will be held on the afternoon of the 15th.
Can also refer to a period of fifteen days (a duration of 15 days), depending on context.
商品の交換は到着から１５日以内にお願いします。
Please request an exchange within 15 days of arrival.
入院は１５日ほどで退院できました。
I was able to leave the hospital after about 15 days.
キャンペーンは申込日から１５日間有効です。
The campaign is valid for 15 days from the application date.