very crude rule whereby a Saturday night stay within the return flight dates can cut the price (of business/first fares especially) by a huge amount

the theory being that business travelers don't stay away on a Saturday night and want to get home...

post-Covid that has flexed a bit for some travel where business trips might be fewer but longer or involve some element of tagging on "leisure" days i.e. a weekend in New York before some work meetings there - but there is still a healthy premium the airlines can charge for the more "true" business travellers (also tend to be more frequent travellers) who really just need the classic mid-week trip: regular business travelers are already away enough that they are not interested in staying away any Saturday or other nights more than necessary, and that reflects in the pricing they can be charged (which they also don't actually pay for themselves, generally)