Hi All!
We were just married on August 20th! We stayed in Cabo from the 15th-29th. We did have an outdoors ceremony, and reception. Ceremony didn't start until 6pm. Sunset was around 7:50pm.
I will say that every day there it was 37-40 degrees celcius, and very humid.
We got used to it after a couple days.
The wedding was fabulous, and as long as you don't mind your hair going flat after a couple hours, you will LOVE it!
We did have fans out for the ceremony, and everyone used them!
We were supposed to get married in May, but had to change dates after the swine flu thing. I googled the actual temp there for May when we were supposed to get married, and it was the same as it was in August. The weather will be what it is. I wouldn't stress about it!
As for rain, we asked how often it rains in Cabo, and were told 2-3 days per year. Usually rains in late August... it rained the very last day we were there (29th).
Good luck on your planning, and regardless of when you go, you will have the time of your life!