Unification Day (Bulgarian: Ден на Съединението, romanized: Den na Saedinenieto) on 6 September is a national holiday of Bulgaria.

By the terms of the Treaty of Berlin (1878), Southern Bulgaria (named Eastern Roumelia) was separated from the newly formed Bulgarian state and returned to the Ottoman Empire with partial autonomy.

At first the plan was to annex all territories that Bulgaria had gained after the Treaty of San Stefano but later it became clear that was impossible because of the unsuitable international situation.

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".

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.