Moshood Kashimawo Olawale Abiola (24 August
1937 – 7 July 1998), often referred to as M. K. O.
Abiola , was a popular Nigerian Yoruba businessman,
publisher, politician and aristocrat of the Yoruba Egba
clan. He ran for the presidency in 1993, and is widely
regarded as the presumed winner of the inconclusive
election since no official final results were announced.
He died in 1998, after being denied victory when the
entire election results were dubiously annulled by the
preceding military president Ibrahim Babangida
because of alleged evidence that they were corrupt and
unfair.
Since then, Nigerians has used this day to honour him and the southwestern states in reference to his memorial has always declared June 12 as a public holiday
www.wikipedia.com/mkoabiola