De-thatching

Heavy thatch
If the thatch layer builds up to a certain level, it tends to compress and become ´hydrophobic,´ meaning it blocks water movement. It can also prevent the ability of fertilizers and pesticides from reaching the intended targets. Generally speaking, a thatch layer greater than 1/4 inch can cause enough problems to warrant removal. 

Is thatch bad for a lawn?
Yes. Too much thatch is not good. If left untreated, thatch can develop into a nearly impenetrable disease-harboring mat between grass and the soil. Thatch sounds scarier than it is, for it can be removed quickly and easily using a hand rake. Once the initial thatch layer is removed, future problems can be avoided by following proper fertilizing and watering procedures.

Aeration

Lawn aeration involves the removal of hundreds of small soil "plugs" from a lawn. A machine extracts cores of soil roughly 1/2" in diameter by 1" to 3" long as it is driven or pushed over the turf. The holes poked into the turf provide a direct path for air, water, and nutrients to reach the grass roots. These soil plugs, which can be raked over the surface of the turf, will break down, thus accelerating thatch decomposition and the return of nutrients to the soil.

Spring or Fall?
Fall is usually the best time to aerate. The reason for this is that aeration usually exposes a lot of soil. Doing so in spring means exposing soil to warm season weed seeds like crabgrass. Crabgrass needs both heat and light to germinate. Once it does, it can quickly dominate a lawn with so much bare soil to work with. Crabgrass and other warm season weeds typically die off with the first frost. If you aerate in the fall, you may still get some warm season invaders, but the upcoming cool weather will take care of them.

Overseeding

Overseeding, or adding new seeds to existing lawns, improves bare spots, thickens turf and helps crowd out weeds.

Best time to Overseeding
Early Fall is the best month for starting grass seed. The soil is more consistently warm than in the spring and the new grass will have a big advantage against competing weed seeds next spring.