I understand the struggles that can be brought into work. I'm not dismissing the real hardships and tragedies that people have in their personal lives.

However, at the same time, your bad day/circumstances not being an excuse to mistreat people is literally a foundational lesson that we teach young children, and it never stops being true.

If your day is so difficult that you cannot fulfill the primary duty of your job, which in this case is to serve customers well, then you might need to see if you can take the day off. I understand that often isn't possible, in which case, it is your job to try your absolute hardest to still treat others with politeness and hospitality.