At the beginning of the movie, George says that he was on his way home "on a perfect fall day" when Annie comes home from Rome. The wedding date is January 6, which Frank says is 5 months away. This would put the "perfect fall day" somewhere at the beginning of August. Not too fall like to me. Especially in California!
Actually George says he his coming home "on a perfect fall day" in Father of the Bride 2 not 1. In Father of the Bride 1 it is in the summer when Annie comes home.