A bad day never hath a good night.

Trouble during the day can linger after dark, leaving a person with little relief when evening comes.

The saying treats a difficult day as something that casts a shadow over the night, rather than ending neatly at sundown. Its absolute wording gives force to that feeling.

Use it when an upsetting or exhausting day continues to weigh on someone in the evening, or when daytime troubles carry on into the night. It expresses a gloomy expectation, not a rule that every bad day must end badly.