You are what you think about.
Think about the last time you were having a so called "bad day". Identify the sequences of events, and more importantly, the resulting thoughts you had from the events, that led up to you concluding you were having a "bad day".
What you will find is, when something happens to us that we don't like, we attached a bad thought or feeling to it. And conversely, when something happens to us that we like, we attached a good (or positive) thought or feeling to it.
This process just described puts us at the mercy of events and things that happen to us. If more bad things are happening to us than good things on a particular day; we think, feel, and say "I'm having a bad day". And the opposite is also true.
The problem is, what if you have something very important that you want to do or accomplish and your having a so called "bad day". Does your important accomplishment, by default, get automatically get included in the "bad day" your already having?
I'll show you how you can avoid this from happening. First you must start separating feelings and thoughts from events, and let your thoughts and resulting feelings STAND ON THEIR OWN, independent of any event.
When thoughts and resulting feelings are attached to an event, the event can, and will dictate your feelings. So if you are feeling bad as a result of negative events it will be impossible to perform at you best level at anything you endeavor to accomplish. I have witnessed people going through incredibly difficult events, but they seemed to still have JOY and inner peace. They may not have been happy, but they had an uncompromising joy and exuberance for life that couldn't be quenched.
Happiness is very different than JOY. Happiness IS based on what's happening to you - but - JOY isn't. For example: You're not going to be happy if you find out you have cancer, but you can have the knowledge that you are still the same person before and after the bad news, and that you also still possess the same intelligence, talent, charm, personality, desire and will before and after the bad news.
Allowing an event to affect your JOY and the essence of who you are is a CHOICE. It's YOUR CHOICE. Don't let an event or another person make that choice for you.
I use the cancer example because I've met people that are battling cancer right now, and I'm amazed at how positive these people are about life and about themselves. You see, these people have not let their circumstance or negative events in their life; define who they are, or steal their JOY of living, or diminish their desire to accomplish things.
If you string together enough bad thoughts as a result of negative feelings derived from negative things that happen to you, I guarantee that you will end up concluding "you are having a bad day"