A referendum on joining the European Economic Community was held in Denmark on 2 October 1972. The result was 63.3% in favour with a turnout of 90.1%. The law that Denmark should be member of the EEC was passed on 11 October 1972, and Denmark became a member on 1 January 1973.
Background
According to Article 20, section 2 of the Danish constitution, any law that makes limitations to the sovereignty of the Danish state (as membership of the EEC would) must be passed in the Danish parliament with 5/6 of the parliament's members voting for the law. If a majority of members vote for the law, but not by 5/6 majority, and the government wishes to uphold the suggested law, the law can still be passed in a public referendum, as was the case in the 1972 referendum.