For purpose of discussion, assume that due to bad luck, this asteroid has evaded the detection of all amateur and professional astronomers until about six months from impact. The asteroid is too large to deflect with humanity's current spacefaring capabilities, and the general scientific consensus is that the impact will end all multicellular life on Earth.
What do those six months look like?
The novel doesn't describe the event itself as much more than flashes of light in the sky, but apparently McCarthy did a bunch of research on what would happen to the earth in the event of a major impact before writinf the novel, and the descriptions of the sun being blocked out, the sea turning grey etc i think would fit with that