September in Fairfax is the sunniest month of the year. In September, the weather in Fairfax is typically warm. Average daily high temperature in September is about 24 degrees above zero. On average, Fairfax has 15 sunny, 8 partly cloudy, 6 rainy, and 1 cloudy day in September. The rains are usually moderate. September as well as October is considered the most comfortable time to visit Fairfax.