Today is the last day of the exhibition.
I have to submit the report by the final day.
最終日 is the last day of an event, the direct opposite of 初日, the event's first day.
最終日 means 'the last day' (of an event or period), while 終日 means 'all day long'; the two share the characters 終 and 日 but differ in meaning.
Compound of 最終 (final, last) and 日 (day).