A year after the Russo-Turkish liberation war, a Constituent Assembly was convened in Veliko Tarnovo. Some the main tasks were the drawing up of a constitution and the selection of the capital of the Principality of Bulgaria. On April 3, 1879, the members of the Constituent Assembly unanimously elected Sofia as the capital of the Bulgarian state. This commemorative date was solemnly celebrated in 1979 - the 100th anniversary of the proclamation of Sofia as Capital of Bulgaria. In 2016 the day was celebrated in Banski Square for the first time - in front of Sofia History museum. This year, a series of events will be held from 29 March to 29 May in honor of the 140th anniversary. The culmination of the celebrations will be on April 3, when at 11 am in Alexander Nevsky Square an official ceremony of consecration and giving flags to schools in the capital will take place. Residents and guests of the capital will have the opportunity to join the special tour "Behind the Facades of Sofia" for free and learn more about life in Sofia during the first decades after April 3, 1879. At 6 pm Sofia Wind Orchestra will hold a special concert at the Largo. A series of concerts of the ensembles of the Municipal Cultural Institutes will be performed every Wednesday at 6 pm in April and May at the Largo. From April 1 to April 15 at the Lovers’ Bridge will be presented two outdoor exhibitions – the Photos of Sofia Exhibition and another one organized by the State Archive. A retro streetcar will travel along the central streets of Sofia in honor of its 20th anniversary.
You can find more information here.