Overseeding a fescue lawn in the spring is tempting. The weather is cool, your fescue lawn is green and rain showers are consistent. All are ingredients for a good time to aerate and overseed. Your seed will germinate and appear to go gangbusters.
What most people do not consider is how the transitioning weather from spring to fall will affect newly sprouted fescue seedlings. Fescue is considered a cool season grass. Once temperatures rise and summer droughts set in, mature fescue effectively goes dormant. New seedlings, with young root systems will not be able to tolerate the summer heat and dye off. The likelihood of the new grass coming back to life post-summer weather is very low.
Therefore, the best time to overseed with fescue is in the fall. This allows the new seedlings to mature over the fall, winter and spring seasons, thus giving it enough time to become strong enough to tolerate the next summer's weather.