That day has been appointed by a decree of the Ministerial Council dated May 31st 1993. It was first celebrated back in 1884 in Vratsa and Plovdiv. It was first officially celebrated in 1901 when the celebrations up on the Vola Peak where Botev found his death were attended by some of his fellow freedom fighters from his own detachment.
In the period 1953-1988, marked by communism, that day was celebrated as the day of Botev and All Who Lost their Lives in the Fights against Ottoman Opression, Capitalism and Fascism and in the War of Liberation. After several more names, the day finally received its current one in 1993.
The photo above: the monument of Hristo Botev at the Tsar Simeon Garden (also known as the Central Park of Plovdiv)