Weather Report: Go to Sicily, then Capri and Herculaneum, in Sept or Oct?

Of course as you said you can't predict the weether, but in 2015 I was in Capri and the area around Herculaneum the first week in October and had pretty good weather. We had one day of off and on rain and one cloudy day. The rest of the time was sunny and really nice temperatures. I don't remember what temperature, but I wore capris and short sleeves every day, and it was even warm enough to get in the water on the coast. Some evenings we wore light jackets. We went on a boat ride when we were on Capri and it was a really beautiful time for it. Whenever you decide to go, I hope you have a great trip!

I wouldn't worry about rough seas and rain can happen anytime, including August. Early September can be really hot however, while early October has more amenable temperatures. If this is primarily a beach going vacation, early September might be better, but if the purpose of your trip is visiting historic sights etc, you might be better off in early October when temperatures are less debilitatingly hot. Rainy weather tends to come toward the latter part of October and early November.

I have been to Southern Italy multiple times over the years in Sept. and Oct. Early September pretty much means beach weather...I visited this year in early Oct. and it was just tons of rain, but large ferries, the kind that take on cars and freight always cross. Three years ago early Oct. was beach weather and very hot...so you never know as others have said. If it is a dream once in a lifetime trip try to make the Sept. dates work is my best advice. Enjoy!