Almost, I think, but not quite. In the Basics 2 tips it mentions the following:
"Estas varma nokto = It is a hot night.
Note: Do NOT say "Ĝi estas varma nokto". Ĝi (it) in Esperanto always stands for a noun, but in this sentence it doesn't stand for anything at all. The subject of the sentence is varma nokto."
I think "Estas bona tago" would be acceptable.
According to Wiktionary, you need the accusative: http://en.wiktionary.org/wiki/bonan_tagon#Esperanto
Thank you! I guess I just have to learn more languages to see more connections; at the moment I can only try to find parallel words in English and Latin. Update: Ooooh, now I get it. "Tag" is implanted in"mittagessen". I didn't realize that when I was simply memorizing the words :)