I think one of the greatest things about Esperanto is that it's super easy to learn. I think this is one of the main reasons it was able to spread so much. People don't give up on learning it like they would for Russian or German, because it's so easy to learn. This is by far the language with THE most straightforward grammar ever.
- And because English grammar is not understood well by most native speakers, anyway.
Usually we go by what "sounds right".
- And because learning languages often helps with similar vocabulary from the same language family.
(for this purpose English & Esperanto are almost in both the Romance family & the Germanic family).
- And because learning a second language always helps with learning other languages - confidence, perserverance, techniques, etc
When I started taking Spanish freshman year of high school, I became really glad English shares so many latin cognates with it. Thank to the Normans for invading England in 1066. You guys made my Spanih class 900 years later so much more easy. On the other hand, German and Dutch might've been easier for English speakers if they hadn't invaded and Old English hadn't lost so much of it's inflection and West Germanic grammar. Altho I suppose the Viking invasion of England is to blame for that as well.
"Bonan tagon" is short for "Havu bonan tagon." ("Have a good day.") or "Mi deziras al vi bonan tagon." ("I wish you a good day.") Therefore it is a direct object and gets the -n.
The same goes for other short noun phrases in Esperanto. "(Mi deziras al vi) Gratulon." / "(I wish you) congratulations."
Here is the section of the grammar book PMEG about that, but it's in Esperanto: http://bertilow.com/pmeg/gramatiko/frazospecoj/mallongigitaj_frazoj.html#i-tfd
Tagon is just the accusative form of tago. It's the same word and the same meaning. You have to use the accusative because you're wishing it. It's as if you wrote "Mi deziras bonan tagon al vi" (I wish you a good afternoon". If you write "Bona tago!", it means that you believe that today is a nice day, not that you are wishing a nice day to someone.
Every language and culture divides the day in its own way. In Canada we almost never day "Good Day"". It's either "Good Morning" or "Good Afternoon" .In Spanish "Buenos días" literally ''good day" is used from the morning until the early afternoon; after that it become "Buenas tardes". In Esperanto you have ''Bonan matenon"" (Good Morning), Bonan tagon ( Good day, good afternoon) and Bonan vesperon (Good evening). So, for "Bonan tagon", bothe Good day, and good afternoon should be accepted.