On Sunday Bulgaria is celebrating the 130th anniversary of an event which a number of historians argue should be the most important national holiday.

September 6, 1885 was the day when the Principality of Bulgaria (Knyazhestvo Bulgaria, which comprised most of present-day Northern and Western Bulgaria) and the then-Ottoman province of Eastern Rumelia (Southern Bulgaria) in the autumn of 1885.

Co-ordinated by the Bulgarian Secret Central Revolutionary Committee (BSCRC). The Unification was accomplished after revolts in Eastern Rumelian towns, followed by a coup on 18 September (old-style 6 September) 1885 supported by the Bulgarian Knyaz Alexander I.

The Unification itself was proclaimed in Plovdiv, back then the main city of Eastern Rumelia - and this is where the biggest events take place every year to mark the anniversary.

A concert is due to begin on Plovdiv's main Unification (Saedinenie) Square starting 12:30 EEST: just a prelude to the culmination due in the evening. At 20:00, a ceremonial inspection of the Armed Forces of the Republic of Bulgaria will be conducted by President Rosen Plevneliev (Commander-in-Chief under the Constitution) and will be followed by fireworks.

As in previous years, several museums are offering free entry on Sunday. In Sofia these include the National Historical Museum and the National Museum of Military History, and in Krumovo, near Plovdiv the Aviation Museum is opening its doors to all citizens and guests.

Other cities and towns across Bulgaria will mark the anniversary with various events, parades and ceremonies.

Even though the liberation of Bulgaria from Ottoman rule (March 3) is now the country's most important national holiday, many insist this is how September 6 should be designated instead - the Unification has remained the sole major event in Bulgarian history when political success was achieved through domestic, nationwide efforts, rather than being brokered from outside.

March 3 for its part is the date of the Treaty of San Stefano, signed after the 1877-1878 Russo-Turkish War which gave birth to a much larger Bulgaria but which also stationed Russian troops on its territory and was followed by another treaty splitting the nation into the Principality of Bulgaria and Eastern Rumelia.