You don’t want to apply weed killer spray to that lawn went the temperature is going to be 85° or above at any time during the several days after the application or it can kill the grass in addition to the weeds. The lawn should also be well hydrated (moist soil) during that period as well. When the grass is dehydrated, the weed killer can damage or kill it as well. The best time to go after weeds is September when the soil is cooler and moisture but it is warm enough for the weeds to be killed. More on late summer weed killer HERE.