Rita, I have had several horses, even mature ones (4, 5, & 12 yrs old) gelded in summer. It may not be the ideal time, but I had bought them as stallions with the intent that they be geldings, so I didn't want to wait and deal with a stallion for the rest of the summer. The vet didn't seem concerned about the timing, and we didn't have any complications.
Where I am in SW OK, the fly problem really isn't that bad. I know in other parts of the country that the flies are horrendous, so I don't know if it would work to geld during fly season in those areas, but the flies reallly weren't a problem for me.