Here in NC they say to seed around Labor day or after, so you should be good to go. One thing about dethatching though is if you already have fescue you really shouldn't need to do it unless you've had large areas that died and have thatch layers covering the ground. Otherwise, you're just gonna be pulling up good grass plants...not good. You might find the link helpful also. Here is a link that might be useful: linky

ranger, To put it into perspective, I did an overseed about 5 weeks ago. I used Tall Fescue and Kentucky Bluegrass, and overseeded into a 10-month old PR/KBG lawn, going against common advice in so doing (I actually prefer lawn with the mixture of all three types, but maybe I'm the odd one out). And it did work, contrary to what people have said. In fact, it worked quite well. The existing PR did not prevent germination of the other grasses. However, I the lawn drastically prior to seeding, as you can tell, via aeration (three passes) and verticutting (two passes). All the TF is up now. The KBG is about a week into germination at this point; I'm just starting to see it come up over the past few days. I'm currently touching up the few spots that need it with a little extra seed. Here are some photos of the project (same area plus or minus a foot or two) at the various stages for reference (I do consider it a successful outcome even if it was my first time doing it and I learned a lot!): The top photo is the before photo, taken on 8/17/12. The middle photo is about a week after overseeding, 8/29/12. And the bottom photo is from 9/21/12, immediately after the first mowing and application of start fertilizer (I decided to hold off on the fertilizer until after germination). Good luck!

Catkin--With PR's fast establishment and fine fescue's slow growth, mowing lower than normal should be fine. Perennial rye is a bunch grass and so are all the fine fescues except for creeping red fescue. So the only grass in your lawn that will spread at all is the CRF. This is not a function of the soil, but a function of the grasses in your lawn. Bunch grasses don't spread even when the soil is healthy. If you've got enough CRF in the fine fescue mix you put in last fall, it may spread enough to keep the lawn full once you get it into good shape to start. CRF doesn't spread as aggressively as KBG does, so I think your plan to overseed now is probably a good idea. In the future, if the CRF doesn't make up enough of the mixture, you may need to overseed again. Fine fescue doesn't germinate quite as fast as rye, but it germinates much faster than KBG so if you need to overseed in the future, you can probably still avoid scalping, but you'll probably want to mow lower to give the new seeds a chance to establish before the existing grass needs to be mowed. Good luck, and keep us posted on your progress.

If you can isolate the spots where you added seed and water those for sprout, that would be great. But you probably can't, so simply keep the watering going for another 7 to 14 days. After that, once the new seed sprouts, you can cut back.