With an outdoor wedding you always should have a back up plan because of rain. I would say have it at the beginning of Oct. or end of Sept. you can have your beautiful colors and still have it warm. If you are a member of a church you can usually get the building for free, so you could book that as a back up plan. Or use your reception site. A friend had her wedding in Oct. and it was calling for rain so they decided to have it inside since they set up the day before and there back up was at the hotel where there reception was (it ended up being a warm day but she said it was still beautiful)