It's too young. If it were a fall planting, I might suggest judicious use if you wanted to take the risk, but a summer planted lawn is stressed already and won't put up with that easily. At this point, you're stuck until September. For particularly obnoxious spots of weeds, you could spot-spray, but definitely expect losses in the grass that's in there as well.

Seeding a northern lawn in June is a mistake. The only worse month is July and early August. Fall is the best time to seed for you. Do you know what seed they used? The options are fescue, rye, and Kentucky bluegrass. The only weeds that will die are the summer annual plants like crabgrass. They leave a ton of seed behind for next year, but with proper watering and mowing that threat will be minimal. So here's what to do. Keep watering and mowing the weeds as if they were the perfect lawn. No weed killer yet. The idea here is to get roots deep down into your soil. Roots and water are the best things you can do to improve your soil. Deep and infrequent is the mantra for watering. Simply going to 1/2 inch of water, 2x per week, will get rid of some weeds that need continual water. Normally I would say 1 inch all at one time, but your roots are still young. Mulch mow at the mower's highest setting. Tall grass has deeper roots, and you want that. Tall grass will shade out some of the weeds, so that is a good thing. At the end of August, evaluate what you have. If you have a blanket of grass with a few weeds, then you might not need to reseed. Spot spray the broadleaf weeds with Weed-b-Gon. Give that a week or so to work and fertilize on Labor Day. But if you have a blanket of weeds with a little grass, then you may as well nuke the entire yard with RoundUp and reseed. Even if you nuke the yard you are not wasting your time mowing and watering now. You really need the roots in the ground. I won't get into reseeding unless you know you need it. A picture would help us to help you. The best pictures are taken on cloudy days or when the Sun is low in the sky so the yard is in the shade.

It is both a weed and a grass. Any plant you have that you don't want is a weed, by definition. Getting rid of grassy weeds in grass is usually a matter of pulling the weed by hand. There are selective herbicides but they select for grasses or broadleaf plants. There are some herbicides that can target grasses in a grass lawn, but it depends on the grass you have. What would that be? And go ahead and tell us where you live. The zone doesn't help.

That does look likely. I was going to say a Commelina sp.