Don't know about Boston area, but here in NJ this was a very bad summer
for disease. There are many lawns in my area that suffered severe
damage. I had some problem areas as well, most likely pythium blight
and brown patch. The grass in a few areas just collapsed and died in
the first two days of 90+ heat.

I'd start by getting the soil tested and adjust the PH if needed. Are
you sure you have a thatch problem? Thatch is a thick layer of
undecayed grass matter that is so dense water has a difficult time
penetrating. It is most common on bluegrass and usually not a problem
with fescues.

Now is the best time to aerate and overseed. If the grass has been a
problem for sometime, is of poor quality, disease prone, etc, you may
want to kill it all off and reseed the whole thing. You can do that
with Roundup and reseed about 10 days later.

Forget about doing it by hand. You can get quotes on having it done,
but I think you'll find you can do it yourself for a fraction of the
cost. You can rent a core aerator for $50 for a half day. Same for a
slice seeder. I'd aerate first, then use the slice seeder. While
you can just spread the seed around after aerating, using a slice
seeder is far more effective. It cuts slits and gets the seed into
them, assuring good germination.

Make sure you choose the right type of grass for the location and
conditions. Buy only the best quality seed. There are many new
varieties of really superior seed available today. After seeding, put
down starter fertilizer and keep it constantly damp for 2-3 weeks.