From IMDB:
#1 - 6 December 1979
#2 - 4 June 1982
#3 - 1 June 1984
#4 - 26 November 1986
#5 - 9 June 1989
#6 - 6 December 1991
#7 - 18 November 1994
#8 - 22 November 1996
#9 - 11 December 1998
So, while the last few have been Nov/Dec releases, Trek has been a summer movie...