17th day of the month
17 days
「一七日」 has another reading with different meanings.
十七日の午後に面接がある。
There is an interview on the afternoon of the seventeenth.
来月の十七日に届く予定だ。
It is expected to arrive on the seventeenth of next month.
十七日は祝日なので学校は休みだ。
The seventeenth is a holiday, so school is closed.