Weed killer and grass reseed
The interval that you need to wait before putting down grass seed varies and depends on what the active ingredients are in the fomulation of weed killer that you use. Herbicide labels will list this on the label.
The type and amount of weeds that you have determines what weed killer is best for you to use. Take a looks at our Lawn Weed Gallery to determine what are your worst offenders here: http://extension.umd.edu/hgic/lawns/lawn-weed-identification
This link also discusses control options.
We are not sure what you mean by 'selective' lawn weed killer. Do you mean one that kills broad-leaved weeds only but leaves grass alone? That is our usual understanding, but often lawns also contain difficult perennial grassy weeds like nimblewill or Japanese stiltgrass, or even annual crabgrass. 'Selective' broad-leafed herbicides would not work on them and you would need to use a non-selective herbicide containing the active ingredient glyphosate to kill those grasses back and then re-seed those areas. Round-Up and other brand names contain glyphosate, but there are now many mixtures with other additions. Some allow seeding within 3 days, others have a longer period, and some particularly difficult weeds may need a second application. Again you need to identify the weeds and read the label for the products that you find in your store.
You have plenty of time, as we are just coming up to the best time of year to seed turf-type tall fescue.
Generally if you have more than about 40% weeds you should consider doing a lawn renovation. Here is more information on that: http://extension.umd.edu/hgic/lawns/lawn-renovation
To get detailed information on how to go about lawn overseeding and renovation be sure to read through
this publication: http://extension.umd.edu/sites/extension.umd.edu/files/_images/programs/hgic/Publications/HG102%20La...