An evening designates a gathering of people, invited or not by an organizer in the same place, in order to socialize, to speak or to entertain. The characteristic elements of an evening are the presence of food and drinks; They are usually accompanied by music and dance.
An evening is not necessarily private. Public evenings, most often paying, can be organized in bars or pubs.
Most of the time evenings are organized but they can just as well be improvised.