noun
13th day of the month
Used to specify the 13th day of any month. Often appears in dates like 13日(じゅうさんにち).
会議は13日に行われます。
The meeting will be held on the 13th.
noun
13 days
Used to express a duration of 13 days. Often followed by a counter or in phrases like 13日間.
この仕事は13日かかりました。
This work took 13 days.