"July 1... Opens up a completely new perspective, a new world for us," Prime Minister Zoran Milanovic told parliament on Saturday, denying Croatia would be a burden on the EU.
Croatia will become only the second former Yugoslav republic to join the bloc, after Slovenia, following the bloody breakup of the ex-communist federation in the 1990s.
In a symbolic gesture to celebrate membership, Croatia will remove the "Customs" sign at a border crossing with Slovenia at midnight on Sunday.
The two events will be broadcast live in a Zagreb square, the main site for celebrations expected to be attended by more than 100 European dignitaries.
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Heads of state from all six ex-Yugoslav republics will be among the guests, but the leaders of many EU member states including Britain, France and notably Germany will not be present.
German Chancellor Angela Merkel cited a full workload to excuse herself from the event, but both Croatian media and the opposition labelled her decision a "diplomatic slap".
The EU anthem, Beethoven's "Ode to Joy", will be played,and then several dignitaries, notably European Commission President Jose Manuel Barroso and Croatian President Ivo Josipovic, will address an expected crowd of thousands.
The Balkan country has aspired to join the EU club ever since it proclaimed independence from the former Yugoslavia, a move that sparked the bloody 1991-1995 war against rebel Serbs backed by Belgrade.
Social Democrat premier Milanovic has dismissed fears the country of 4.2 million would be an economic burden for the bloc.
"At the end of the day we are not part of eurozone," he said, referring to the crisis-ridden single currency bloc.
Milanovic's centre-left government hopes that EU entry will attract badly needed foreign investment and boost the economy with 11.7 billion euros (USD 15 billion) of potential financial aid.
Croatia will be the first new member since Bulgaria and Romania joined in 2007, but the celebrations are undoubtedly overshadowed by economic worries.