I know the basics of a few different languages, and I get the sense that you either say in those languages "Good morning", or "Good day" when you are greeting someone, which seems to be the case: "Bonan tagon KAJ BONVENON".
Now when it comes to English, as far as I know, you can say "good day", but at least nowadays, not as a form of saying hello, but rather when parting, especially if you're annoyed, and you're saying it sarcastically.
So what's the rule here? You say "Bonan matenon" in the morning, or you can also say "Bonan tagon" instead, and use it throughout the whole day in general, until sunset? Is that it?
Bonan tagon = hello (all day). It's like bonjour (French) or Guten Tag (German). Bonan matenon = good morning.
"Have a good day" in US English, or "I wish you a good day" in British English. But "havu" and "deziras" take the accusitive.
LaPirocque: In this instance, bonan tagon is the shortened version of the sentence I wish you a good day -- good day is an object, so it needs the two -n. This is covered in one of the "Tips and notes" sections. (I've been told that one can see them when using a laptop or desktop but not when using a mobile device.)
Bonan nokton = good night. Bonan vesperon = good evening. Bonan tagon = good day (litteral translation but the better traduction would be hello, guten Tag in Deutsch, Bonjour in french, Buenos dias in Spanish...)
What is it that you don't see? The matter mentioned by GreanThom about "Good day and welcome" being short for "Have a good day and welcome", perhaps? The fact remains that such greetings as "Good Day", "Goodnight", "Happy Christmas" and "Happy New Year" are all contractions, with "Have a..." or perhaps "I wish you a...." missed out. That is why all such greetings take the accusative -n in Esperanto. Of course the same words can be used in sentences where they are not a greeting, and in such cases the accusative may not be needed. For instance, "It's a good day for the competition" would be "Estas bona tago por la konkurso", but greetings, and expressions like "Bonan apetiton" (literally, "Good appetite", or as we would say in English, "Enjoy your meal") always take the accusative.
It's actually short for something like, "Mi deziras al vi bonan tagon" . ("I wish you a good day"). The English expression, "Have a good time" is another way of saying, "Enjoy yourself!" which in Esperanto would be: "Amuziĝu!"
"Good day" (almenaù cxi tie en Britio) estas uzata anakromisme. Nuntempe, oni multe pli ofte uzas: "Good morning", "Good afternoon", ktp, aù malpli formale, "Hello", aù ecx pli malformale, "Hi!"