Gotcha, trust me I have read a ton of posts, could only find what everyone is spraying, not really when. Mainly treating the winter annuals, dandelions, and clover. Not treating anything like sedges, crabgrass, etc.Depends on the weed. Do you mean crabgrass? Winter annuals? Dandelions? Perennials? Summer annuals?
Usually it is best to spray when the weed is young and growing rapidly. That depends on the weed species and the weather--different every year.
Educate yourself. Go to turf conferences. Talk to dealers. Read our old posts.
Beyond that I cannot help you, as I am too far away--you need local advice.