Per its use in many TV shows and movies, I gather that the “that’s the night I wash my hair” excuse was a stereotypical one that women gave for not agreeing to go out with a guy, at least through the 1970s. Maybe home blow dryers weren’t a widespread thing till the '80s. I know my friends and I would joke about it in college (I went '80 to '84) when the topic came up of “getting shot down” by the object of one’s romantic interest: “Did she tell you she was washing her hair that night?”