The best period for visiting Sicily is from April to June and late September to October the temperatures are mild and you would find fewer crowds.
August is probably the worse month to visit Sicily its extremely hot and overcrowded during the peak tourist season, especially around Ferragosto in mid-August.
November is one of the best month, it's low season which means less pricey and less tourists.
During winter, Sicily is cool with increased rainfall amounts, but the rain isn't necessarily continuous. You will have the occasion the skiing in the Etna volcano overlooking the seaside.