21st day of the month
Used to specify the 21st day of any month in dates.
会議は２１日に行われます。
The meeting will be held on the 21st.
21 days
Indicates a duration of 21 days. Often used with counters or in expressions of time span.
この薬は２１日間飲み続けてください。
Please continue taking this medicine for 21 days.