your plan should be to replace weeds with turf grass and keep the turf alive so it doesn't turn to weeds again and need never ending chemicals and labor.
Fall is the best time to plant turf but if you spray regular broadleaf weed killer you have to wait 4-5 weeks after to put seeds, and shouldn't spray a heat stressed lawn and since this year was severe drought and heat the lawns are still very heat stressed and I'd wait a bit before they bounce back some in order to spray weed killer but by then it might be too late to seed grass depending on what dates are shown to be the best time to plant in your area, but you'll be fine with a later date especially if you don't have much leaf competition falling on the new seedlings and if have full sunny areas go for the later date and once it starts growing teaspoon feed it starter fertilizer, plus use some organic slow release at the start like milorganite or purely organic lawn food (Actually a decent deal even with free shipping from home depot).
You can also use Tenacity/Meso at the time of seeding and it won't affect your seeds but it doesn't do the greatest job at killing weeds like a common 3 way or especially triclopyr ester.
Any grassy weeds like crab and foxtail you don't have to worry about it competing with your new seeds because they're basically done growing by then, prevent them with pre emergent in late Winter. You can use Drive XLR8 to kill them though and seed right away usually depending on what type of grass you're planting as per the PDF.
Most other bradleaf etc doesn't sprout in Fall, only Spring, so no need to worry about those now but Tenacity also has a pre emergent affect but only lasts about 6 weeks.
Poa grasses do sprout in Fall though and they're basically like turf grass but can't take any heat at all, it looks like cool season turf but once heat comes regardless of water it'll die back.
If you had a lot of Poa areas and wish to replace them with grass, even Tenacity or any seed-safe starter like Scotts that says prevents weeds but is also a starter fert for new grass will still allow poa to sprout because it's so similar to turf grass. If have lots of poa areas then I'd get a propane torch wand and very carefully on a damp day with hoses and extinguisher near by torch the areas will kill the seeds and then you can plant turf there safely.
www.canr.msu.edu
