From Longman Dictionary of Contemporary Englishbad hair dayˌbad ˈhair day noun [countable] informal a day when you are unhappy and easily upset, especially because your hair does not look the way you want it to look
Examples from the Corpus
bad hair day• Your basic bad hair day at the photo lab.• I felt miserable and realised the hair of my dreams had turned into the worst bad hair day you could imagine.bad hair daybad hair daya day when your hair does not look tidy or neat even when you try to arrange it carefully – used humorously I’m having a bit of a bad hair day. → hairExamples from the Corpus
bad hair day• Your basic bad hair day at the photo lab.• I felt miserable and realised the hair of my dreams had turned into the worst bad hair day you could imagine. 