I'm not as interested in the answer to that question as in how I formulate my answer. What criteria do I use to judge whether a day is good or bad? How do I measure a day, what 'rulers' do I use. A few possible answers.
Have I had any meaningful conversations with anyone today?
Have I spent any time being creative?
Have I avoided negative emotions/experiences and maximized positive ones?
Did I feel connected with God today?
There are just some of my rulers, and unfortunately they are often in conflict with one another. I need time alone to be creative, but if I maximize that alone time then I avoid meaningful conversations with other people. The act of creativity can sometimes feel negative for me, some works of art have very painful births. If creativity leads me to encounter negative emotions I would rather avoid then not being creative makes it a good day. I've had times when my encounter with the Divine has left me feeling empty and convicted rather than full and peaceful.
I've used many different rulers over the years. In high school a good day was measured by what classes I had scheduled or whether I successfully avoided the school bullies. Some rulers have been discarded while others have become their exact opposites. A good day in high school used to be one where I didn't have to go to P.E. Now, a good day is one where I make it to the gym.
Every ruler that I use reduces the item being measured to less than its totality. A meal can be measured by Taste, Texture, Appearance, Aroma and Nutrition. For years I avoided bananas solely because of their texture, it was only this year when I finally peeled and consumed my first one, and now I eat at least one a day. For decades I missed the bananas' benefits because I only evaluated them on texture.
Every day we are bombarded with an over abundance of experiences and emotions. Measuring and categorizing helps us order them and prevents us from being overwhelmed. Measuring also minimizes and distances ourselves from our own lives, it is difficult to evaluate and experience at the same time.
Some of my bad days in retrospect are some of my best.