The weather in Positano shows all the pleasantness of its Mediterranean climate, with mild winters and hot (but not too hot) summers.   Because of this, the window for travel is quite large.   From May to October, the weather is quite hospitable, involving average highs in the 70s and 80s and lows in the 50s and 60s.   If you’re looking for heat July and August have the highest average temperatures (ranging around 85 degrees).   Rain is fairly light in Positano with October –January being the rainy months.     

There are several festivals in Positano, including various summer music celebrations (both Jazz and classical music), as well as the August 15 th Feast of Assumption (the most important religious festival of the year) which features a mock pirate attack on the beach, firework displays, and of course, feasting.  

Positano also hosts various concerts, film festivals (both the Cartoons on the Bay and the Vittorio de Sica festival), dance awards (the Positano Prize), and fashion shows (Moda Mare in June), as well as many traditional celebrations (including lavish Christmas and New Years revelry and a Fish festival).  

For more information about the weather and when to go, please refer to the following links:
http://www.positanonline.it/welcome_e...
http://goeurope.about.com/od/positano...
http://www.fodors.com/miniguides/mgre...