This is going to be a casual wedding. Please bring something nice for the wedding ceremony, but just dress comfortably for the rest of the weekend.
For the ceremony:
Ladies - think sundresses and comfortable shoes (bring your dancing shoes but stilettos won't work so well in the outdoor setting).
Gents - dress it up a bit, but no need for a full suit and tie. A nice guayabera will also work well (click here for pics).
Carmel Valley Weather
Carmel Valley is the warmest spot in Monterey County. That said, it still gets cool in the evenings. The average temperatures for September range from the mid 70s during the day to the mid 50s at night.
Dress/Attire