Have a nice day. Have a safe flight. The yearbook standard, HAGS. Get better. Even sleep well.
In English when we want to wish someone well we often command that well of them. We treat the good tidings like they are in that person’s control. (This phenomenon may also appear in other languages—I am not well enough tongued.)
Likely we are simply contracting “I hope that you have a nice day,” or the like—and I am just observing the overlap of subjunctive and imperative—though I am curious of speakers’ awareness that they are so contracting.
My question is one of history. Have we always been comfortable with this elision? At what point did it become acceptable, if at one point it was considered rude, to command someone to be well, rather than to wish it upon them?
I could believe a correlation with secularizarion, as hope or prayer becomes less useful or sensical without a sense of a divine power—in what do we hope? Instead we make the hearer of our well wishes an agent of their own well-being, rather than the recipient of any mercy.
Secondarily I am interested in whether we can now consider the construction an imperative, even if it once was the contraction of a subjunctive phrase.