Sofia's 6th September Street was named after one of the most important events in Bulgaria's history - the unification of the northern and southern parts of the country, the Principality of Bulgaria (Knyazhestvo Bulgaria) and Eastern Rumelia (Iztochna Rumelia). The day has been declared an official holiday and is celebrated in all parts of Bulgaria.

After almost five centuries of Ottoman rule, Bulgaria officially gained its independence on March 3, 1878 with the signing of the San Stefano Treaty in Turkey. According to the treaty, Bulgaria became an autonomous vassal principality with a Christian government, its own administration and army. The state borders included the region of South Dobrudja, the northern and southern parts of today's Bulgaria, the whole of Thrace (without the Odrin and Gyumyurdzina regions), the whole of Macedonia (without the Thessaloniki region and the Halkidiki Peninsula) and Pirot. That way the new state included most lands inhabited by Bulgarian people.

The European powers, however, were alarmed that the newly liberated state would become a dependent of Russia and make the Russian Empire even stronger. So, they summoned a congress and signed the Berlin Treaty in 1878, according to which the northern part of Bulgaria and the Sofia region became an autonomous principality called Knyazhestvo Bulgaria and the southern part of the state remained a province under Turkish sovereignty named Eastern Rumelia. The province enjoyed considerable autonomy and was ruled by a governor appointed by the Ottoman Empire with the approval of the European powers.

Masses of people in the province of Eastern Rumelia, comprising the lands of Upper Thrace, which were inhabited entirely by Bulgarians, did not passively accept the decisions of the Treaty of Berlin and were engaged in a powerful movement for their unification with the principality. They would not allow any Ottoman troops to come into the province, comprised its administration and army only of Bulgarians, and confined the powers of the Turkish government to the walls of its own building.

In April 1885, a Bulgarian Secret Central Revolutionary Committee was formed in the province's capital of Plovdiv. Its mission was to organize and carry out the unification of the Principality of Bulgaria and Eastern Rumelia. The committee leaders accepted a plan of action, according to which a unified liberation campaign with the participation of guerrilla fighters and Rumelia police would be carried out. The political leaders of the movement came into direct contact with political parties in the Principality of Bulgaria and the Bulgarian monarch, Prince Alexander Battenberg. After certain hesitation, the prince took the side of the liberation movement. On August 30, he was informed by the committee about upcoming military actions and ordered that the Bulgarian army was kept in constant fighting form.

The unification campaign started in mid-August with public anti-Turkish demonstrations. On September 6, officers from the Rumelia police arrested the province's governor, Gavril Krustevich, and the government was deprived of power. An interim cabinet was formed and a total mobilization was announced.

According to the action plan of the unification campaign, armed troops flocked to Plovdiv but actual battles did not occur. On September 8, Prince Alexander issued a manifest which officially accepted the unification and annexed the former province to the territories of the Principality of Bulgaria. The next day the monarch went to Plovdiv and was welcomed with official celebrations by the cheering population. The unification provoked negative reactions from the European powers, especially Russia. With the annexing of the new territories, Alexander also angered Serbia, which claimed that area.

The Ottoman Empire was not eager to accept the unification act but finally agreed to name Prince Alexander governor of Eastern Rumelia. The province officially became independent in 1908.

September 6 is one of the least controversial dates in Bulgaria's modern history and is widely celebrated every year.