1The 21st day of a month (used for dates).
締め切りは５月２１日です。
The deadline is May 21st.
２１日は映画が安くなりますか？
Are movies cheaper on the 21st?
彼の帰国は７月２１日になる予定です。
He is scheduled to return to the country on July 21st.
2A duration of twenty-one days (a period lasting 21 days).
研修は２１日間の予定です。
The training is scheduled for twenty-one days.
彼女は２１日連続で自習した。
She studied on her own for twenty-one days straight.
薬は２１日間服用してください。
Please take the medicine for twenty-one days.