noun
14th day of the month
Used to specify the 14th day in dates. The reading じゅうよっか is irregular; note the small っ.
会議は十四日に行われます。
The meeting will be held on the 14th.
来月の十四日までに提出してください。
Please submit it by the 14th of next month.
noun
14 days; two weeks
Used for a duration of 14 days. Often interchangeable with 二週間, but 十四日 can emphasize the exact number of days.
この薬は十四日間飲み続けてください。
Please continue taking this medicine for 14 days.
二週間 is the more common way to say 'two weeks' as a duration. 十四日 emphasizes the exact count of 14 days and is less common for durations.
Compound of 十 (じゅう, ten), 四 (よん, four), and 日 (か, day). The reading よっか is an irregular sound shift from よん + か.