Your bad day doesn’t owe anyone a smile. Being polite and honest is enough.

Cheerfulness is not a duty you owe strangers. On a difficult day you can be polite, do your work, answer questions and still not perform a mood you do not have. The effort of faking it sits on top of everything you are already carrying, and it is usually the part that finishes you off.

Make plain and civil your setting on a rough day: please, thank you, clear answers, nothing extra. If someone asks why you are quiet, saying it is a hard day is a complete answer. Save the real conversation for somebody who has earned it.