Ceremonies will officially start with an opening concert. Commission President Jean-Claude Juncker will deliver a speech along with Bulgarian Prime Minister Boyko Borissov.
Also travelling to Sofia are Council President Donald Tusk and his Parliament counterpart, Antonio Tajani.
On Friday, Commissioners will participate in meetings with the Bulgarian government to discuss the main priorities of the presidency.
These focus on three themes: external relations, security and defence, migration and justice; an inclusive and sustainable Europe closer to the citizens; and a competitive, innovative and digital Europe.
Ahead of the ceremonies, the Greens/EFA group has said Bulgaria takes on the presidency at a “testing time” for the EU.
The group’s co-leader Ska Keller said, “We are hoping that by the end of the presidency, Europe will have taken a step forward. Whether it is safeguarding the future of the eurozone, creating a more genuinely social Europe, getting real on fighting climate change or solving the rule of law crisis in several states, they have a difficult but important job to do.
“One of the key underlying problems in Europe is the lack of trust and solidarity between member states, exemplified by the debate on the reform of the asylum system and namely the Dublin regulation. It is high time for the new presidency to get moving.
“The European Parliament has already set out ambitious proposals that would help bring an end to the terrible human suffering caused by the broken system. The new presidency should make steering this through the European Council a top priority.
“Bulgaria also has to clean up its act at home, where corruption has been tolerated for far too long. Their plans to open up the Pirin National Park to commercial logging are a further cause for concern and I would urge the government to listen to the voice of the protesters and change course,” said the German MEP.
Further comment came from group co-leader Philippe Lamberts who noted, “Unfortunately, European negotiators failed to clinch a deal on the long-awaited revision of the posting of workers directive during their last-ditch meeting before the holidays. The Bulgarian presidency will inherit this important file. Despite its weak support of the new provisions, we expect Bulgaria to act as an honest broker and not undermine the attempts made so far to improve the protection and rights of the nearly two million workers concerned.
“While the European pillar of social rights was acclaimed as a landmark to address the concerns of the most precarious citizens and to restore positive outlook among the population towards the European project, this roadmap should be fleshed out in the coming months, notably on the occasion of the new cycle of the European semester and the adoption of the country-specific recommendations.”
He added, “It should also be used as a guide to fix the functioning and the governance of the eurozone.”