At times it has also been common for airlines to charge lower prices for round trip tickets that extend over a Saturday. The thinking being that business travelers usually return home on Thursday or Friday and they're less price sensitive so airlines could use that as a way to discriminate. Leisure travelers typically stayed through a weekend and received lower prices.
My anecdotal experience is that this sort of discount doesn't really exist any longer. At least it isn't obvious to me when I book for the most part. Saturday night stay discounts for flights used to be very pronounced.
