Every day's a school day!
Why were they racing on a Saturday?
I was preparing an answer that would apply to British GPs. They were held on Saturdays so as not to disturb church services up until the mid-80s.
However I’m not sure why Las Vegas 1981 or South Africa 1983 were, though I’d assume a similar reason.
Japan 1987 was on a Sunday, but Mansell crashed in Q1, making Piquet the champion on Friday.