Perhaps annual bluegrass, technical name Poa annua.

If so, a very common lawn invader.

Might be Annual Bluegrass, although it tends to grow in more of a clump, or it might be Annual Ryegrass. Both are cool weather grasses that die out in the hot summer weather and are consiered "weeds" by many, although both are sold as cover crops.
The method of ridding your lawn of either of these would also kill off your lawn is you chose to apply something. Since shortly they will doe off anyway I would not concern myself too much and concentrate of keep them from making seeds and encouraging the grass I want to grow in strong and healthy and thick to discourage any seeds produced form germinating this fall.

Okay, Thank you, but how would I encourage the grass' that I want? Over Seeding?

From a capitalist's perspective: what you desire is a monopoly for the grass variety of preference. It will take effort to eliminate the competition. Certain fertilizers include poisons which allow your particular lawn type to prevail, but the most certain approach is to sit down with a hand tool and dig them out while you still can.

When I bought this house, the lawn was pure evil. I found remnants of an earlier lawn, and just dug out everything else. It took a while, but my lawn is postcard quality now.

The longer you delay, the worse things will become. Help your lawn to beat the competition!

You don't have to subscribe to the monoculture requirement for mowed areas in your own yard unless you want to.