Today, one of my colleagues, who sit at the desk in front of mine, decided to have a bad day. She got really upset about her chair being misplaced as someone borrowed it last afternoon. I understand that having your belongings (though own by the company) used when you are not here is not pleasant.
Then it was her computer. Then it was the noise. Then it was the people around her who are not nice enough. Then it was the work load. Then it was not going to eat outside of the company enough (when we go outside often she complains about her weight). It is only mid day, but she already found about a dozen reasons to complain loudly to everyone and have a bad day. Including about facts that happened weeks ago and belong to the past.
Shit days happen to everyone and for a variety of reasons. You have the right to be upset by events and thus have a bad day. Consciously turning your day in a shitty one is completely different though.
I am rarely in a bad mood (usually when I didn’t sleep enough the night before). Even those days, I’d rather isolate myself from my colleagues (meaning I have my headphones on and won’t talk much to people) rather than have them suffer my bad mood. I also try to smile more than I’m inclined to on one of those days. Thus, my colleagues tend to smile back at me and be nicer. My mood usually improves after a few hours (and coffees), and I go back to having a better day.
I also tell people I am a bit grumpy that day, so they can stay away from me or won’t be offended if I’m not my usual self. If I'm at home, I'll spend some time alone and rest, or do things that make me happy for a while before I'm a social being again.
Besides, your day won’t be better/easier/shorter if you spend it complaining and looking cross… So don’t forget to smile! It won’t fix all your issues but will improve your mood!