Everybody has a bad day once in a while. Things don't go as planned. You're tired or sick or frazzled and just not up to your usual standards. Your mood takes a dip and you're more irritable than usual.
"Bad Days" are nothing to be ashamed of. If you were a machine, you would run, run, run until you broke, but as a human being you have emotions, moods, fluctuations in energy, distractions, limited control of reality, and of course, stress. The occasional bad day is to be expected.
But if your bad days turn into bad weeks, that may be a sign that something is wrong. It's one thing to be depressed for a couple of days. It's another thing to be depressed for more days than not over a two-week period. Depression is not something to take lightly. Depression costs you life.
If you've had one bad day too many, maybe it's time to talk to someone. Maybe something is going on with you, and maybe your bad days are a signal that your emotional health needs attention.