Bom, essa é mais uma daquelas expressões que se ouve por ai nos filmes. É uma expressão informal americana de acordo com o dicionário de expressões idiomáticas de Michaelis.
Seu significado é “estar naqueles dias que tudo dá errado”.
O Dicionário de MacMillan diz:
Noun [count usually singular] INFORMAL
1 a day when your hair looks messy and you do not feel attractive.
2 a difficult day when nothing seems to go well for you.
O NTC’s American Idioms Dictionary de Richard A. Spears PhD:
1. A day when one is unable to arrange one’s hair into a satisfactory and pleasing style.
2. Used figuratively for a problematic and difficult day
Vejamos alguns exemplos:
Ex.: I’m sorry I am so glum. This has been a real bad hair day.
Ex.: It’s just one bad hair day after another.
Ex.: Yeowch! I’m having a total bad hair day!
Ex.: Many people believe that there’s nothing worse than a bad hair day.
Ex.: It’s not the end of the world if you have a bad hair day.
Ex.: So if Americans have a “bad hair day,” they have no one to blame but themselves.
Ex.: I’m having a bad hair day today, so I’m wearing a hat.
Ex.: I’m having a bad hair day.