That's the hardest "general knowledge" quiz I've ever taken on this site. I use quotes because this stuff obviously isn't generally known if the average is only 7/20.
Texas was the last slavery holdout. When the troops liberated the slaves in Galveston on June 19, slavery was eradicated in the US. Having said that, we all know why Juneteenth became a federal holiday.
The Wikipedia page for the topic has a few idioms listed for other cultures. Apparently the term used in Japan would translate to "oops-we-did-it-marriage".
Guess it's not that hard any more. I don't run the site, but if it were up to me, I would leave the hard quizzes and just put the new questions on a new quiz so there were even more quizzes available to take, there were challenging quizzes with informative questions, and the old stats wouldn't be reset.