alt Hacker NewsThe difference between phrases and "words with spaces" is addressed.
The confusion might be that this seems to be a spectrum rather than a binary phenomon.
We have single words at one extreme, ordinary sentences at the other, and in the middle we have idiomatic assemblies of words that span a range of substitutability.
"Hot dog" and "Saturday night" are arguably great examples, because they exist at the opposite extremes of the spectrum. Saturday night can retain some of the original meaning following substitution, whereas hot dog almost deserves a hyphen.
I disagree that "saturday night" ever means anything other than the literal meaning of the nighttime of the day of saturday.
You can argue that there's a connotative association with the phrase. Sure. Just like "beach weather", or "blizzard conditions". But that doesn't make "saturday night" special in any way.