To maintain a healthy green, and weed- and disease-free lawn, it should be fertilized 4 times a year. Weeds and disease thrive on poorly nourished soil.
If you mulch and cut your lawn often enough, grass and leaves generate enough nutrients to eliminate one fertilizing.
Step one in most fertilizing programs calls for a pre-emergent weed controller to kill the hardy weed, crabgrass. Crabgrass is a patchy, low growing weed. Once you apply the weed killer, you cannot plant or grow grass seed for 6 weeks.
If you don’t have any crabgrass, I suggest Milorganite for the first feeding. It will not burn grass seed if you want to do a little over-seeding to the yard.
Step two is to apply a topical weed and feed. To work properly, apply it to a wet or damp lawn and stay off it for 24 hours. It needs the moisture to cling to the broad leaf weeds to kill them.
Step three is to use only fertilizer to keep the lawn mean and green.
Step four is to create a stronger lawn using more potash to make the grass winter and drought resistant.
There are certain weeds, like creeping Charlie, clover and other vine-like weeds, that require a special weed killer.  I like Ortho’s Clover and Chickweed killer for these rare but pesky weeds. It only comes in a concentrate that you can jar spray or mix to spray in a tank sprayer.