Looks like annual bluegrass to me... http://www.msuturfweeds.net/details/_/annual_bluegrass_47/ I do not think there is an easy way to control it, other than pulling it by hand or waiting for it to die with the heat. I am kind of new to this world and hopefully I am wrong, let's see what other more knowledgeable have to say Here is a link that might be useful: msu annual bluegrass

Yep. Whether Poa Annua or another bluegrass that is typical this time of year. It is having a banner year due to the longer winter and cool spring. It likes cool and damp and if it is the Poa Annua then heat kills it off. Don't water until you must and wait for the heat. A pre-emergent for crabgrass control in fall and early spring next season will knock it way back but those seeeds last years and nothing kills it that doesn't kill your turf as well. Also, next year if it starts popping up weeks before your lawn turn you can add vinegar to a pump sprayer and spray it and every 'early' weed before your grass greens up early in the day then the sun/acid will burn it right up before it spreads and goes to seed. What kind of (intentional) lawn do you have? Once the soil warms to around 70, especially if fairly dry and not shaded, this stuff dies out.

Yeah looks like Triv, which is tougher to remove. Where did you get your seed? Was it a sun shade mix? I read on another forum that Poa Triv is added for shade tolerance in some big box store brands. Tenacity will whiten Poa Triv but not kill easily, I don't think. Is that all you have? If yes dig it up with weed removal tool and get every last bit of root. if you have turf type tall fescue do not use Certainty , you may need Velocity which recently became available for residential use.

Campanula rapunculoides. It is highly invasive, both by spreading or creeping and by the massive numbers of seeds it produces. Dave's list calls for full sun, but it is not fussy, either about soil or exposure. It stands up to weed killers and if dug out, it almost certainly leaves lots of seeds behind as well as any little piece of root forming a new patch. It generally takes years of careful and thorough digging and spraying to clear it and it will still invade off the back alley or from a neighbors neglected flower bed. It gives me headaches here in Minneapolis and has for years.