The earliest Eurovision date was 3 March, in 1957 and the latest 29 May, in 1999.

March April May
03. 1957 03. 1971, 1976 03. 1986, 1997
05. 1966 04. 1981 04 1985, 1991
11. 1959 06. 1968, 1974 05. 1984, 1990
12. 1958 07. 1973 06. 1989
17. 1962 08. 1967 07. 1977
18. 1961 09. 1980 09. 1987, 1992, 1998
20. 1965 22. 1978 10. 2007 (s)
21. 1964, 1970 23. 1983 12. 2001, 2004 (s), 2007 (f), 2009 (s1)
22. 1975 24. 1982 13. 1993, 1995, 2000
23. 1963 30. 1988, 1994 14.
2009 (s2)
25. 1960, 1972 15. 2004 (f)
29. 1969 16.
2009 (f)
31. 1979 18. 1996, 2006 (s)
19. 2005 (s)
20. 2006 (f), 2008 (s1)
21. 2005 (f)
22. 2008 (s2)
24. 1956, 2003, 2008 (f)
25. 2002, 2010 (s1)
27. 2010 (s2)
29. 1999, 2010 (f)
(f) = final (s) = semi-final
(f) = final (s) = semi-final

In the Rules of the 2008 Eurovision Song Contest, it was clearly stated that:

“Eurovision Song Contest (the “ESC”) is produced each May under the auspices of the European Broadcasting Union (the EBU)”

The word “each” may have been inserted with no intention to influence future contests but, I’m clinging to it.

Eurovision Song Contest DatesSpringEurovision LyricsThe earliest Eurovision date was 3 March, in 1957 and the latest 29 May, in 1999. March April May 03. 1957 03. 1971, 1976 03. 1986, 1997 05. 1966 04. 1981 04 1985, 1991 11. 1959 06. 1968, 1974 05. 1984, 1990 12. 1958 07. 1973 06. 1989 17. 1962 08. 1967 07. 1977 18. 1961 09. 1980 09. 1987, 1992, 1998 20. 1965 22. 1978 10. 2007 (s) 21. 1964, 1970 23. 1983 12. 2001, 2004 (s), 2007 (f), 2009 (s1) 22. 1975 24. 1982 13. 1993, 1995, 2000 23. 1963 30. 1988, 1994 14. 2009 (s2) 25. 1960, 1972 15. 2004 (f) 29. 1969 16. 2009 (f) 31. 1979 18. 1996, 2006 (s) 19. 2005 (s) 20. 2006 (f), 2008 (s1) 21. 2005...Profesionalni prevodi vaših omiljenih pesama. Translation of Serbian, Croatian, Bosnian, ex-YU lyrics, poems etc. All Eurovision Lyrics