Neilson is a patronymic surname meaning "son of Neil".
 The prefix "Neil-" is of Gaelic origin, a genitive of 'Niall', possibly meaning "Champion" or "Cloud".
 Neilson, as a surname, is common in English language-speaking countries, but less common as a given name.
 It can be an anglicisation of MacNeil (Gaelic: MacNeill) or the Scandinavian name, Nielsen.
The Neil Kerley Medal (formerly known as the Trabilsie Medal and later the Steve Hamra Medal) is an Australian rules football award given to the player(s) from the West Adelaide Football Club deemed Best & Fairest for the season.
 West Adelaide play in the South Australian National Football League.
Country of origin (COO), is the country of manufacture, production, or growth where an article or product comes from.
 There are differing rules of origin under various national laws and international treaties.
 Country of origin labelling is also known as "place-based branding", the "made-in image" or the "nationality bias."
 In some regions or industries, country of origin labelling may adopt unique local terms such as terroir used to describe wine appellations based on the specific region where grapes are grown and wine manufactured.
Donald Neil Kerley (born 20 February 1934 in Barmera, South Australia) is a former Australian rules football player and coach.
 Known to his mates as either Neil or Knuckles and to South Australian football fans as King Kerley or simply "Kerls", he is best known for taking three clubs to four South Australian National Football League (SANFL) premierships over three decades as both a player and coach and for playing 32 state games for South Australia.
 Despite his first name being Donald, it was his mother Lillian's strong dislike of shortening Christian names that led everyone calling him Neil and not Don or Donald from an early age.
Ian Douglas Day (born 9 February 1935) is a former Australian rules footballer who played for West Adelaide between 1952 and 1960, and South Adelaide between 1961 and 1964.
 He captained the team in 1963 and played in the 1964 Grand Final premiership under captain Neil Kerley, which was the last premiership to be won by South Adelaide.
Damien Nygaard (born 24 May 1945) is a former Australian sportsman.
 He played 96 SANFL matches with Norwood and made three appearances for South Australia between 1964 and 1969.
 Nygaard is probably best remembered by most for his collision with Glenelg hard-man Neil Kerley in 1969 where he sustained a broken jaw.
Neil is a masculine given name of Gaelic origin.
 The name is an Anglicisation of the Irish "Niall" which is of disputed derivation.
 The Irish name may be derived from words meaning "cloud", "passionate", or "champion".
 As a surname, Neil is traced back to Niall of the Nine Hostages who was an Irish king and eponymous ancestor of the Uí Néill and MacNeil kindred.
 Most authorities cite the meaning of Neil in the context of a surname as meaning "champion".
Denis O'Neil (born 11 February 1936) is a sailor from Australia.
 O'Neil represented his country at the 1972 Summer Olympics in Kiel.
 O'Neil took 16th place in the Soling with Robert Miller as helmsman and Ken Berkeley as fellow crew member.
 During the 1976 Summer Olympics in Kingston, Ontario O'Neil returned to the Olympic regatta again in the Soling.
 with David Forbes as helmsman and John Anderson as fellow crew member O'Neil took 11th place.
Rules of origin are used to determine the country of origin of a product for purposes of international trade.
 There are two common types of rules of origin depending upon application, the preferential and non-preferential rules of origin (19 CFR 102).
 The exact rules vary from country to country, from agreement to agreement.
A homeland ( "country of origin" and native land) is the concept of the place (cultural geography) with which an ethnic group holds a long history and a deep cultural association – the country in which a particular national identity began.
 As a common noun, homeland, it simply connotes the country of one's origin.
 When used as a proper noun, the Homeland, as well as its equivalents in other languages, often have ethnic nationalist connotations.
 A homeland may also be referred to as a "fatherland", a "motherland", or a "mother country", depending on the culture and language of the nationality in question.

Question: What country of origin does Ian Day and Neil Kerley have in common?
Answer: Australian