"According to tradition there will be an excursion on Wednesday."
Translation:Laŭ tradicio okazos ekskurso merkrede.
Only sort of. Esti doesn't take an accusative because it's kind of like an equals sign. There's no action of the verb.
- Mi estas viro.
- La pomo estas rugxa.
Okazi and ekzisti don't have objects. In fact, in this sentence, the reason "ekskurso" doesn't have an -n on it is that it's the subject. "Ekskurso okazos" - an excursion will happen.