1. A day when things go wrong, when one experiences misfortune, difficulties, or negative events
I had a bad day at work; everything went wrong.
Tive um dia mau no trabalho; tudo deu errado.
2. A day when someone feels sad, upset, or in a poor mood
She's having a bad day, so let's give her some space.
Ela está tendo um dia mau, então vamos deixá-la em paz.