A day fighter is a fighter aircraft equipped only to fight during the day. More specifically refers to a multi-purpose aircraft that does not include equipment for fighting at night, radar, although it is sometimes used to refer to some interceptors as well.
Examples of planes that were classified as day fighters were the Supermarine Spitfire and Messerschmitt Bf 109. Both were excellent interceptors, but were also found in roles such as strike fighter and reconnaissance. Yet neither was really able to be used as a useful night fighter, so the term day fighter was used to describe designs of this sort.
Today almost all fighter aircraft have enough equipment to be used as a night fighter, so the term has fallen from use.
The last of the day fighters were the F-16 and the YF-17 that competed for the Light Weight Fighter contract which had the goal of providing the USAF with an inexpensive day fighter that could be purchased in great numbers to establish air superiority. Ironically, the F-16 after winning the contract quickly matured into a much more capable multi-role aircraft.
See also:
night fighter, heavy fighter, interceptor aircraft