It is very unlikely that you have a thatch problem this soon after a new lawn intallation. If you can post some pictures you may get an answer that may save you a lot of time and money. BTW a machine that dethatches is called a power rake--same thing. The process of making holes in the lawn is aeration and the machine is called an aerator. Plug aeration is the most recommended type.

You want to mow the lawn as short as possible before seeding. You do that ti help increase the seed to soil contact and also to ensure the existing grass doesn't need to be mowed before the new grass can take mowing. If you want to keep the existing grass, mow it shorter in stages, lowering the deck gradually so that you remove no more than 1/3 of the height of the grass each time you mow. If you don't care whether the existing lawn lives, you can drop the deck all at once (but the mower may bog down a lot). When you're scalping the lawn, you want to bag the clippings. Other times, you want to mulch mow. If you core aerate, I don't think you need to dethatch. I've never had a problem with thatch (mulch mowing and infrequent watering helps keep the thatch layer to a minimum). Make sure the soil is damp several inches deep before aerating. After aerating, overseed. You should try to get this all done while you still have about 6 weeks before you expect frost. If you miss that window, your next best option is to dormant seed (seed in the winter for spring germination). If you dormant seed, the aeration (or dethatching if the thatch layer is really bad and the soil doesn't really need aerating) should be in the fall.

Organic fertilizer begins "releasing" nitrogen immediately in the form of protein being fed to microbes. What takes roughly 3 weeks is the soil food web processing where the microbes feed on other microbes and their waste products to eventually release a nitrogenous plant food. Similar to the way mammals, birds, reptiles, and fish excrete a nitrogenous plant food, there are microbes which do the same thing only it is more tuned in to the needs of the plants. If you want to target a date where the fertilizer effect is seen, 3 weeks in the future is a pretty good date. Morph is fairly careful when he gives dates for applying organic (early May) versus chemical (Memorial Day) fertilizer. I'm less careful. I picked the federal holidays as application dates more because they are easy to remember. I took a couple years making notes about the yellowness of my grass in San Antonio. Those dates kept my grass green all season.

@gle2011: I think shallow is relative. Some grasses have roots 2 and 3 ft deep. If you mow your KBG at 4" the roots will be about 4". If you mow it at 2" your roots will be about 2". I think the idea is to maximize the root depth by mowing high and encouraging the roots to reach down where the water is, not curl up and form a thatch mat while looking for shallow water. I think your site is vague about how dry the surface might become before needing more water. A lot of us have looked around and see our neighbors watering every day, doing everything wrong, yet having none of that brown straw mixed in with the green blades. If the search feature of this forum weren't so pathetic, I might be able to find my post on this subject. If I recall correctly, morpheus said that you might have to give up a little green in the summer heat but you gain resistance to many other problems. Personally, I have yet to see one of these neighbors' lawns crash and burn due to poor watering. On the other hand, I lost a few patches to fungus while doing everything "right"! I'm going to water an inch once a week whether it needs it or not next season to observe any difference.