Ok, so we are just now setting our date, and i've always wanted an outdoor wedding in the fall time.., I live in Arkansas so the weather is always crazy and changes here... so really don't know what month to choose..
Having a back-up option is good. Check the farmer's almanac for typical weather during your date. If you really want an outdoor wedding stick with it. You can usually rent a tent last minute, although it is pricey.
Re: I just don't know?? :/
Or, depending on your locale, you can rent portable heaters for weather issues.