Unification Day (pronounced as "Saedinenie" in BG) on September 6 is a national holiday of Bulgaria. It commemorates the unification of Eastern Rumelia and Bulgaria in 1885.
According to the Berlin Treaty from 1878 Southern Bulgaria, named Eastern Roumelia is separated from the newly formed Bulgarian state and returned to the Ottoman Empire with partial autonomy. The Bulgarian citizens considered the decisions of the Berlin Treaty to be unfair and began a peaceful demonstration against them.
At first the plan was to annex all territories that were torn from San Stefano, Bulgaria but later it became clear that was impossible because of the unsuitable international situation. The Bulgarians had to leave Macedonia and the rest of Thrace and concentrate on the East Roumelian issue. The first actions were taken in 1880 but the sharpened political situation, economic instability and the low prestige of the Bulgarian country delayed the resolution of the so-called "national question".
In 1884 the "Macedonian committees" were the main body working on the unsolved problem. On the 10th of February 1885 led by Zahari Stoyanov a group of former revolutionaries founded in Plovdiv (the capitol of Eastern Roumelia) a secret committee known as BSCRC - Bulgarian Secret Central Revolutionary Committee (Bulgarian:?????). It had regulations and programs which were connected to the organisation led by Vasil Levski , Hristo Botev and Lyuben Karavelov before the Liberation.
After a few months BSCRC had improved their plan for actions and organised some public events on dates significant to Bulgarians. The conference in Dermendare (Parvanets) on the 24th-26th of July and the next meeting on the 23rd of August declared what should be done and exactly how to procede. On 5 September 1885 the people rose in Goliamo Konare (Saedinenie) and after arresting the prefect of Plodiv the militia advanced toward the capitol. On the 6th of September the palace in Plovdiv was surrounded.
The governor Gavril Krastevich didn't alarm the Turks in Istanbul and proclaimed his support of the people's cause. A temporary government headed by Georgi Stranski took control of the situation and armed forces were commanded by major Danail Nikolaev who began preparation for war with the Ottoman Empire.
On the 8th of September knyaz Alexander I received a telegram in the old capitol Tarnovo which he was already expecting. His army was prepared and waiting for a signal. He agreed to become a leader of the Unified Bulgaria. On the next day (9th of September) the knyaz arrived in Plovdiv and the temporary goverment was dismissed. Diplomatic efforts failed and Bulgaria had to defend its interests on the battlefield in the Serbo-Bulgarian War with the Bulgarians victorious.
info: Wikipedia