A referendum on joining the European Union was held in Poland on 7 and 8 June 2003. The proposal was approved by 77.6% of voters. Poland subsequently joined the European Union that year following the ratification of the Treaty of Accession 2003. The country's first European Parliament elections were held in 2004.
Polish European Union membership referendum, 2003 Wikipedia
Do you approve of the Republic of Poland's accession to the European Union?