When did the first eastern european country join European union

The first time some Central and Eastern European countries joined the EU was in 2004. These were: Hungary, Czech Republic, Latvia, Lithuania, Malta, Estonia, Poland, Cyprus, Slovakia and Slovenia.

Two more countries joined in 2007 and this event is usually associated with the 2004 enlargement, as they come from similar region. These were Romania and Bulgaria.

However, if we speak about Eastern Europe in political understanding, we may often find the former East Germany included. In this case, one could say that it was in 1990, when the reunification of East and West Germany had taken place and thus, the area of former East Germany joined the Union, though without increasing the number of member states.