[bagacrap]

My biggest English language peeve of the moment is the rise in prevalence of "have a nice rest of your day", which appears to be a mashup of "have a nice day" and "enjoy the rest of your day". No matter how much I hear it, it sounds disfluent due to the mismatched particle ("a rest of"). What seems like it started as an honest flub has taken root and is now used intentionally and extensively, especially by grocery checkers.

(I argue this rant is on-topic because it's a "weird", i.e. grammatically inconsistent, turn of phrase.

[archduck]

It's no different from "a nice time in the islands," "a runty pick of the litter," etc.

What's probably tripping you up is that "rest" is usually a noun that takes predicative adjective rather than an attributive adjective. It's more common to hear "The rest of my day was nice," but less common to hear "the nice rest of my day."

But despite the ostensible grammaticality of the noun phrase here, frequency still plays a huge role in determining what we deem acceptable, so it's not sufficient to argue as a counterpoint e.g. "a nice rest of your day" is perfectly fine like these other analogously formed noun phrases, checkmate ... Language doesn't work like that.

I'd argue that your point is actually best articulated simply as "it just sounds weird," rather than calling it any kind of flub, grammatical or otherwise.

[euroderf]

"Have a nice day." "No thanks. I have other plans."