Likening the events or impending doom of a day to that of having an itchy butthole which is the uneasy feeling or "itchy feeling" a person may experience during post-defecation that occurs around the posterior aperture of the alimentary canal also known as your butthole. An itchy butthole day occurs when post awakening, things start going terribly wrong and make you feel like you have an actual flaming asshole, but in actuality, the world is just fucking you in the ass causing a similar, searing sensation in your soul.
I got up late and the kids missed the bus, I spilled my coffee when I got out of my car at work and once I got there I got verbally donkey punched by my boss and I could just tell it was going to be an Itchy Butthole Day.