Because "Van este" focuses on "van", and would be translated as "There is evening", or "There are evenings". "Este van" focuses on "este", so does "It is evening".

Estet is accusative – but pay attention to the accents, 'cos "estet" is not the same as "estét". "Estét" is accusative form of the time of the day, when "estet" is referring to a performance or party that takes place in the evening.