The 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.
A 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.