From Longman Dictionary of Contemporary Englishhave an off dayhave an off daySUCCEED#to be less successful or happy than usual, for no particular reason Even the greatest athletes have their off days. → day
Examples from the Corpus
have an off day• They must now get a result against free scoring Glenavon next Saturday and hope Bangor have an off day at Comrades.• His work isn't usually this bad - he must have had an off day.• You will have off days when you are tired or a bit under the weather. 