During the winter, many people sow a temporary, winter grass like Annual Ryegrass over their dormant warm-season grass lawns. Since fescue remains green during the winter, there is no reason to overseed. Thinning fescue lawns should be reseeded with fescue -- not overseeded with annual Ryegrass.
Overseeding with Annual Ryegrass is a horrible idea since the ryegrass will compete with your fescue for nutrients, water, soil, and sunlight. Once hot weather arrives, the ryegrass will die and leave your fescue lawn weaker and thinner than before.
DO NOT OVERSEED FESCUE GRASS