From Longman Dictionary of Contemporary Englishoff day/week etcoff day/week etcBADa day, week etc when you are not doing something as well as you usually do Brian never usually loses his temper – he must be having an off day. → off
Examples from the Corpus
having an off day• Perhaps Beau was having an off day?• Obviously the market is having an off day, and this is a marvellous opportunity for you to double your stake.• They must now get a result against free scoring Glenavon next Saturday and rely on Bangor having an off day at Comrades. 