Matters become worse when we put off mowing, then cut off long lengths of grass and allow these to fall on top of the dead roots. This debris compacts into a mat. New grass roots begin to grow in this mat where there are few nutrients and no protection from the hot sun. The lawn starts to die. Some people fertilize throughout the growing season, but I let clippings boost nutrients.
Some studies show, too, that frequent fertilization promotes fungal diseases.
I do think it's a good idea to fertilize the lawn in the fall with a formula that will encourage root development over the winter. You don't want a lot of top growth at this time. The healthier your lawn, the less trouble you'll have with weeds. Weeds can be pretty visible in late winter before the lawn begins to grow, but I try to be patient and tolerate those that wander into our turf. I try to avoid chemicals, as they can deplete the soil of good microorganisms and earthworms.