Apply a fertilizer and pre-emergent crabgrass control combo in spring.
The optimum time to apply would be when forsythias start blooming in
your area or a little before. For the summer, generally spot weed
control and insect control can be applied as needed. IMO, putting
fertilizer on in hot weather is the source of many lawn disease
problems, as excessive nitrogen helps promote many diseases.

I apply fertilizer again in early and then late fall, which are the
best times.