At this time of year I am often called to look at a lawn that was seeded in late spring/early summer. The concern is that the new lawn is infested with a significant amount of summer annual grassy weeds. These are typically Crabgrass, Goosegrass, and Barnyardgrass. First of all, weeds are inevitable. Most individuals want to blame the seed, when in fact the soil is the culprit. Weed seeds lay dormant for years in soil just waiting for the opportunity to germinate. In the case of summer annuals, the frost will kill them in the fall. Mother nature will remove them for you in a couple of months so be patient. Keep mowing and fertilizing so that your newly seeded lawn grasses will take over when the weed grasses die. There are chemicals( Ex. Acclaim, Tenacity) that will knock them out but at this late date, I am not convinced you will be any further ahead. You can consider applying a crabgrass preventer next Spring as part of a lawn maintenance program to keep them from coming back.
Turf and Forage Industry - Northern & Transitional Climate Zone