FoxtrotUniform:Bonan Paskon!

Which brings me another newbie question.

Why are greetings and exclamations in the accusative: Saluton, Bonan tagon! Felicxan Naskotagon!

I guess it could be short for a missing imperative "Havu bonan tagon". But I don't get why Saluton would be in the accusative.

I guess I don't really need to understand the why. When I was taking Spanish I didn't wonder why adjectives come after the noun.

Cf. Fundamenta Ekzercaro §10:

Zamenhof:Mi deziras al vi bonan tagon, sinjoro! ― Bonan matenon! ― Ĝojan feston! (mi deziras al vi). ― Kia ĝoja festo! (estas hodiaŭ).

Those expressions are indeed elliptical sentences. The same thing happens in most indo-european languages, only that in English it is almost unnoticeable, because case is generally not visible. One example in English where you can indeed see the (oblique) case in an elliptical sentece, is in "Poor me!" instead of "*Poor I!" (already in Latin this was in accusative: me miserum).