Wikipedia
A day fighter is a fighter aircraft equipped only to fight during the day. More specifically, it refers to a multi-purpose aircraft that does not include equipment for fighting at night (such as a radar and specialized avionics), although it is sometimes used to refer to some interceptors as well.
The term is an example of a retronym: before the development of effective dedicated night fighter aircraft early in World War II, in effect, all fighter aircraft that were not specifically modified for night combat were day fighters.
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 fighter-bomber and reconnaissance. However, 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. Other day fighters such as the Hawker Hurricane Mk.IIC and Fw 190 were also employed as night fighters after a fashion. Although they were marginally more successful than the Spitfire and Bf 109, the reason they were preferred in the night-fighting role is that they had wide-track landing gear, rather than the narrow track landing gear that made landing a Spitfire or Bf 109 so difficult, particularly in the dark. As far as actual night combat went, none of these aircraft were particularly successful compared to true night fighters. Some day fighters like the Grumman F6F, Vought F6U and Lockheed P-38 were converted into true night fighters with the addition of aerial radar tracking systems, with some success.
As aircraft grew more complex and capable, and as much equipment (such as radar systems) that are used for blind flying at night can also be used for flying in cloud and other adverse weather, the term "night fighter" has been largely dropped in favor of "all weather fighter", or "all weather aircraft", as the same technology applies to ground attack aircraft and bombers (with the exception of systems for tracking and destroying enemy aircraft in limited visibility). "All weather" simply implies that the aircraft is not reliant on clear weather or daylight to complete its mission, although they are not truly all weather aircraft; most are incapable of flying under severe weather conditions.
As the size and weight of night fighting equipment remained considerable in comparison to airframe size and engine power, the day fighter remained a part of front line air forces into the 1960s. In the immediate post-war era, designs like the North American F-86 Sabre, Mikoyan-Gurevich MiG-15 and Hawker Hunter typified the day fighter role. These were followed by similar supersonic designs in the early 1960s, including the Lockheed F-104 Starfighter and Mikoyan-Gurevich MiG-21.
However, as the size of basic fighters grew, along with their engine power, the relative cost of carrying night fighting equipment fell until most designs carried such equipment. Lightweight fighters without full night equipment such as the F-5 Freedom Fighter/Tiger II are still used by nations which lack the money to acquire advanced fighter aircraft.
The last of the purpose-designed day fighters were the original American General Dynamics F-16 Fighting Falcon and the Northrop YF-17 that competed for the Lightweight 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. The designs originated in a mid-1960s effort known as the "Advanced Day Fighter". Ironically, after winning the contract the F-16 quickly matured into a much more capable multi-role aircraft and can no longer be considered a day fighter. Likewise, the YF-17 evolved into the much larger McDonnell Douglas F/A-18 Hornet, which is used in the 2010s by the United States Navy as the primary all-weather combat aircraft.