Tens of thousands poured into the streets of the Bulgarian capital, turning the huge Alexander Battenberg Square into a sea of red, heart-shaped balloons.
On Sunday, Al Jazeera's correspondent Stephen Cole, speaking from Sofia, said the mood was "one of expectancy balanced with scepticism".
He said: "Generally it's the younger generation, who don't remember the earlier, tougher times, who are more optimistic."
A Eurobarometer poll, published in December, showed 63 per cent of Bulgarians and 69 per cent of Romanians think their countries will largely benefit from EU membership.
Niculai Idu, an analyst with the European Institute of Romania think-tank, said: "Joining the EU will be a guarantee that Romania will continue to evolve in terms of democracy and a market economy."
But while the polls show the majority support accession, continued poverty and worries about competition between members inside the EU have meant not everyone believes their countries will benefit.
A youth holds EU and Romanian flags in Piata
Universitatii Square in Bucharest [AFP]
Elena Pseclevei, a pensioner who turned out with others on December 25 to light candles at the grave of Nicolae Ceausescy, Romania's former president, said: "I don't think joining the EU will have any results, we will live the same way we do now, which is bad."
Alina Mungiu-Pippidi, of the Romanian Academic Society, said: "Romania looks like it is having a very successful transition, but transition doesn't tackle all the ills."
Bulgaria and Romania will be Europe's poorest members, with gross domestic product per capita barely reaching 34 per cent of the European average, according to Eurostat statistics.
Joining the EU will mean access to billions of Euros in subsidies for both countries, but Brussels has made it clear that the two new EU members will be expected to continue to crack down on corruption and organised crime.
Both Bulgaria and Romania are expected to continue substantial economic and judicial reforms. Many within the union are concerned that the new members may become a "back door" into western Europe for organised crime.
In Romania the celebrations over accession will include a party in Revolution Square in its capital, Bucharest, while Bulgaria is to stage a pop concert on Alexander Battenberg square in Sofia, its capital.
The concert is expected to be interrupted for speeches by Georgi Parvanov, the Bulgarian president, Sergei Stanichev, the prime minister and Jose Manuel Barroso, the European Commission president.
Ivan Krastev, a political analyst with the Sofia-based Centre for Liberal Strategies, said that "with the accession of Bulgaria and Romania the region will receive a clear message that enlargement is not dead".
He said the accession of the two countries would "bolster the voices in Europe calling for the enlargement to continue and include countries like Macedonia and Serbia".
But on December 15, with talks on Turkey's accession partially suspended, the EU agreed to observe stricter criteria on membership.
The decision may have major repercussions for other states hopeful to join the EU such as Croatia, which is expected to join the EU by 2010.