and Stormy night-that opens "Wintersmith". The Book begins with a Winter snowstorm and it starts out with the Feegles observing the full snow clouds which to me indicats that the storm is taking place in the light of day. So it was a gray and stormy day--plus a bit later in the same beginning narrative the comment is made that "the light from the windows was coming through snow". However things on the discworld do seem to happen when the weather is rather inclement. (Dwarfs Turning Iron into Glod, etc.)
I don't feel that despite the Narrative Law of the disc that Pratchett necessarily lets that dictate how his books open.