I will reveal a secret to you: all the audio tracks of this course are recordings made vy a real human, and I even talked to him in person during the 100th Universal Congress in Lille this year!
Because that would make the evening the subject of the sentence, rather than the direct object. The full (implied) sentence being translated here is:
[I wish you] a good evening.
I is the subject.
You is the indirect object (the recipient of the direct object). Which leaves
a good evening as the direct object.
Esperanto doesn't have an indefinite article, so the 'a' isn't translated. And since what's left is the direct object, it takes the accusative case, which means adding an '-n' to both the adjective and the noun. Hence,
I hope this helps.
They're cognates. In Mycenaean Greek the word started with a 'w' or 'wh' sound, so: ϝέσπερος, later ἕσπερος, ultimately from PIE wek(ʷ)speros. https://en.wiktionary.org/wiki/Reconstruction:Proto-Indo-European/wek(%CA%B7)speros