And anti-Brussels protests kicked off in the capital on the same night – with furious demonstrators burning EU flags claiming they feel ignored by the Brussels bloc.
But failing to note the tension raging outside, Mr Juncker made a speech at the ceremony marking the beginning of the Bulgarian Presidency to demand citizens be thankful he decided they could join the union.
He said: “Personally, I feel to be a small part of Bulgaria’s remarkable EU journey ever since the day I signed the Accession Treaty in April 2005 – when I was Prime Minister of Luxembourg and holding the Presidency of the European Union.
“I was in a good mood, in an optimistic mood, in a future-oriented mood, because for years as a Prime Minister and as a Finance Minister of Luxembourg I did admire the performances of the Bulgarian society, mainly of the modest part of the Bulgarian society.”
Jean-Claude Juncker was joined by European commissioners, as well as European Parliament President Antonio Tajani and the Council head Donald Tusk.
But 11 protests were held across the city as the ceremony went ahead, as outraged citizens marched with banners and placards before burning EU flags in anger.
Some even chose to don masks of EU bosses, including Mr Junker and German Chancellor Angela Merkel.
Former president Rossen Plevneliev said: “These protests are not a symbol of future political action, but they are telling us something important – the whole political elite has not done its job over the years.”
George Pykov, a Bulgarian who is currently an executive committee member for Young Independence and studying Law at the University of Portsmouth told Express.co.uk: “The EU has no interest in Bulgaria or any Eastern European country.
“Issues that were meant to be solved have not, and I’m pleased to hear that Bulgarians are fighting back against an expansionist and fanatic Union.
“I and many other Bulgarians living in Britain hope that, one day, we will see a Bulgarian exit from the EU to join the UK in an exciting new, independent future.
“It is time for Bulgaria to be free from mob rule.”
The poorest EU state is taking the EU presidency for first time since it joined the bloc 11 years ago.
It comes as Prime Minister Boyko Borissov, a former bodyguard to the Soviet-era Communist dictator, renewed his alls for eurozone and Schengen membership in a push for further integration with the other 26 members.
He said it was not “fair”, therefore, that Bulgaria was denied entry to Schengen, despite defending the EU’s external border with Turkey, and to the euro zone, despite meeting all the economic criteria and having fixed its currency to the euro for 20 years.
Mr Juncker, the EU chief executive, has thrown his weight behind those demands and repeated that position in Sofia on Thursday evening:
“Your place is in Europe and your place is in Schengen and your place is in the euro,” Juncker told him. “We will work for that. The Commission will be by Bulgaria’s side.”