When I began practising as a dentist, I worked in a small town in north Kent. My working week stated on Monday afternoons. So, Monday morning was available for me to do whatever I wanted. I used to have my regular haircuts on Monday mornings at a barber shop owned by Dave. He often cut my hair and always did a good job.
One Monday morning, I entered Dave’s establishment and as Dave was not around I had my hair trimmed by a young lady. She did a good job but handled my head roughly. She knocked my head around as if it were one of those balls that boxers use for training. I am exaggerating a bit, but there is no denying that having this lady cutting my hair was a stressful experience.
Some hours later, I rang Dave to tell him about my recent visit to his shop. I wanted him to know that if his assistant persisted in treating customers the way she did to me, he would lose business. Dave apologised, and then told me that his young lady had had a bd weekend, a row with her boyfriend.
I suppose that Monday was what people call a ‘bad hair day’ for me.
Bad hair day: a bad day : a day with many problems, annoyances, etc. (https://www.merriam-webster.com/dictionary/bad%20hair%20day)