In what country was the Eurovision Song Contest's 50th anniversary held?

Explanation

In 2005, the EBU organised a special show to commemorate the 50th anniversary of the Eurovision Song Contest. The show, entitled "Congratulations" (after Cliff Richard's entry for the UK in 1968) was held in Copenhagen. Many of the singers from the previous 50 years participated. A telephone vote was held to find the most popular song of all times - a vote that was won by ABBA's 1974 winning entry "Waterloo".