Aerating;
Aeration is done to improve root depth and strength,by allowing air and water to reach the soil much faster.
The Aerating Machine has hollow tines that penatrate the soil,leaving about 3/4" Plugs.(Do not rake these from the lawn,they will dissolve in a few weeks on there own,also mowing will break them down.This is food for your lawn.)
Usually it is good to have your lawn aerated in the Spring or in the Fall.
Also if your lawn is Clay ground,it should be done twice in the year.(Spring and Fall)
If your soil is Sandy you need only aerate once in the year.(Spring or Fall)
There are times when you should and shouldn't aerate your lawn;
When it is really hot and dry is not a good time to aerate,and could stress your lawn.
When weeds are a problem,aerating could help the spread and infestation,making weeds worse.(I advise taking care of the weed problems before aerating.)
If you have lawn that has just been sodded,you should wait untill the lawn has established a good rooting system,otherwise you could end up tearing up the rolls of sod.
Dethatching;
Thatch is okay to have,up to a depth of 1/2",anymore than that,your lawn may need dethatched.
Don't get me wrong,thatch is good for your lawn. So to speak, like you applying sun screen to keep from burning in the hot sun,that is what thatch is to your lawn.Thatch helps to keep off the hot sun in times of droughts,water bans,acting like a sun screen to help your lawn cope better,and also helps retain moisture when summers are hot.
To much thatch on the other hand could prevent your lawn from recieving proper air flow,and when you fertilize lawn, fertilizer doesn't get to the soil,also there is a good chance of insects infesting your lawn,and the possiblity of developing diseases in your lawn.
Just like Aerating,Dethatching should be performed,when the conditions are best for your lawn.
When there is a chance of a long dry spell,water ban,or heat waves is not a good time to have these tasks performed.
Usually when there is a sufficient amount of moisture in the ground this is best found in late Spring or early Fall.
Also just like Aerating,if there is more weeds than grass,it is a good idea to take care of the weed problem before Detatching,(as this may help spread the weeds.
Home and Yard Maintenance 
