The day was declared an official holiday in 1998 and is also the day of the city of Plovdiv.
On the same date in 1885, Eastern Rumelia ousted the rule of the Sultan and declared that it would join the Principality of Bulgaria. This was accomplished after the detachment of Chardafon the Great (born Prodan Tishkov) and soldiers led by Major Danail Nikolaev entered the city of Plovdiv, arrested the District Governor of Eastern Rumelia, Gavril Krustevich and established a temporary government. Two days later, on 8th of September, Prince Alexander of Batenberg recognised the unification through a special proclamation and accepted that from this point on he would be named Prince of North and South Bulgaria.
This remarkable historic event gave Bulgaria the memorable motto “United We Stand Strong” that today stands above the main entrance of the Bulgarian National Assembly.
Source: BNT.bg