Fall starts with the falling temperatures of mid-September and lasts as long as the weather stays nice. The annual Fat Tire Festival in Moab coincides with Halloween, but the weather can be pretty cold by then. Winter sees few riders, but the riding can be nice. Although snow can and does occur, many days bring calm air and a warm sun, making riding a treat.
Sandy trails firm up from frost and the crowds are only a memory.
Of course, patches of ice on slickrock and changing weather are factors to be reckoned with. Summer is also a time of few riders. Temperatures above 100 degrees F cook bikers as they search for shade. Some people ride in the evening, but so many factors make that dangerous that it's not recommended. If you're in Moab during summer, try riding in the La Sals.