A rough day refers to a difficult or challenging day, where things do not go as planned and problems arise. It can be used to describe an emotionally trying experience or a series of unfortunate events that make life more difficult.
Their moods are much better after their daily nap, even on rough days. (twiniversity.com)
He says the reason is being maintaining his health during rough days of filming. (fitnessandpower.com)
If you're facing particularly rough days, just know that they'll pass sooner than you think, even if it's hard to keep that perspective in the moment. (thetoddle.com)