A good hair day mean different things to different people. To me it means voluminous, bouncy, curly hair that's not frizzy. It also has to be not too hot and not too windy, so in essence a good hair day also means a good weather day.
Here are some examples of good hair , it's voluminous right?
Julia Roberts long luxurious waves
Chris Hemsworth mid length men's cut
Borat, very nice for a foreigner!
Kim Cattrall and Sarah Jessica Parker, my hair idol
Tell me what makes you feel like you are having a good hair day?