People are often surprised at the extremes of Fes weather.   In general, the rainy season starts in mid-October and lasts until March.   While it does not rain every day, it might rain heavily for periods of a few days followed by a week or so of grey drizzly weather.   In February of this year (2006), there were only a couple days with no precipitation whatsoever.   From May until October, rain is very infrequent.

Temperatures range from the mid to high 30s C (90s F) in June, July, and August to just above freezing in the dead of winter (December, January, and February).   Strangely, there always seems to be 2-3 weeks of gorgeous weather in mid-December and early January.   Spring and Autumn are beautiful in Fes, particularly Spring when the winter rains bring forth scores of wildflowers.   The nearby Middle Atlas mountains are covered in snow most of the winter and stay refreshingly cool when Fes is baking in the August sun.  

Given that most traditional accommodation in Fes (i.e. riads and converted medina homes) is built around an open-air patio, the cold damp winters can be difficult to escape.   Ceilings are generally very high, which keeps rooms cool in the summer, but makes them very difficult to heat in the winter.  
One benefit of visiting Fes in the off season (winter) is avoiding the massive tour groups that clog the streets in high season.   Also, many hotels and airlines offer special deals to boost otherwise slow business.

Keep in mind that the Fes Festival of World Sacred Music takes place every around the last week of May- first week of June.  Accommodation books up months in advance, so if you plan to visit during this period, make your reservations as soon as possible.