Best time for us northern folk to aerate is in the fall. We have an established lawn and DH has a fall schedule of aerating first, then over-seeding (perennial seed varieties) and fertilizing. This should be done between 8/15 and 9/15 although if you're just aerating, you can go a little later than these dates. Do not use a fertilizer with weed killer in it or grass seed will have a problem germinating. The goal is to get the seed germinated before it turns cold. We use a pelleted chicken manure fertilizer with no additives. Our partially shaded lawn has done very well on this regimen and has gotten thicker.
