It is recommended that you wait until after the new grass has grown to be mowed three times. You should not worry about weeds in your freshly planted grass. The weed can always be removed once the grass is well established.
Just don't spray the weed-killer in a can and go get gloves to pull them out. Because what if that stuff gets in grass too? And it probably won't even kill the weeds all the way anyway. It'll probably just get burned on the leaves and stems but not the roots, they'll just regrow. P.S. Hello, newcomer.