let me explain.

life is nothing more than a series of absurd rationalizations for the reactions we have, unconsciously and immediately, in response to the events of reality.

read that sentence through a few more times.
a bad day, therefore, is merely one when the rationalizations we make are made slightly faster, indeed, too quickly, and disallow the right frame of mind from settling in. that's why they last all day - we are always landing ourselves into patterns, both conscious and sub-conscious, and a bad day is yet another pattern - one of subconscious undesireable (i.e. hastily made) rationalizations.
got it?

--lewis