What was said above...
During prime summer growing season the grass can withstand a ton more abuse, and will be much more resistant... During spring, fall and winter it simply can't recover as fast and it's during those times that the grass and lawn will sustain permanent damage and be unable to recover...
This is why many people don't let 'grazing' animals into the pastures until the grass is well established in the late spring... I do it myself, my llamas and goats get locked in the barn once the snow starts melting (February/March) and are kept in the barn until the pastures gets to about 8-10" tall with new spring growth before they are allow to graze, I also take into consideration the amount of rain fall as soggy wet ground with animals tramping it can be very harmful as well... Then come about October they are again locked in the barn until the snow covers the ground to protect it... This prevents the animals from turning the pastures into mud pits and gives the grass a fighting chances to come back, but even then I over seed the pasture every spring as soon as the snow melts to give it a boost...
Doing this allows my pastures to fully maintain my animals from about April to October with no sentimental feed and no bare patches of mud...
Even if you don't get snow, the growth rate of the plants drastically slows during the winter months as does it's ability to recover...