launch windows

I saw on the CBS News page that the launch window for STS-115 extends
from Aug 27th to Sept. 7th. However if Atlantis doesn't fly then,
there are two launch days in October (23rd and 24th I think) and one in
December - the 23rd -and that's it!
I know basically why they have restricted launch windows, because of
the lighting conditions and the limited ISS rendezvous windows anyhow,
but this webpage said the reason for the limited time is because of
temperature restrictions on the ISS... anyone able to explain that to
me?
Also, they (media, and NASA) said a few weeks ago that STS-116
Discovery was hopeful for the December launch window, however with only
one possible day that doesn't seem very worthwhile to even consider
it..more than likely it'll be pushed into next year - obviously
slipping downstream flights as well..