Regardless of the type, it is best to apply weed killers over a period of several days with no precipitation and little wind. A mid-day application is preferred, to get as much sun as possible while avoiding morning dew. Avoid using weed killer during times of drought or stress to the turf.   If you are using a pre-emergent, the best time to spray is early spring, before seeds have the chance to germinate. For post-emergents, it is best to spray when weeds are a growing phase. It will be much harder to control the weeds if they have matured and established large root systems, so getting this application down by mid-summer is generally ideal.