An evening spent enjoying the pleasures a town or city has to offer. Restaurants, theater, dance clubs, etc. It's probably an abbreviation of "an evening (spent) out(side your home)".
Another common expression for the same idea is "out on the town". "They spent their evening out on the town," or "After work, they're going out on the town." (You'd use it even if "the town" is actually a huge city.)
Leon, think of "evening out" as an abbreviated version of "evening (night) out...of our usual do nothing nights". Thus the idea of going to a restaurant, or theatre or dancing per Mr Capp's reponse. Examples might include mom and dad getting away from those duties when grandma shows up to do "baby sitting"... celebrating an anniversary/birthday/event or simply not doing "the same old thing we always do" on a particular evening. A break from the routine and boringly predictable. You agree, Capp? 08Jun16
Also, for the words starting with "et" eg. Et eple = An apple Eplet = The apple
En Lampe = A lamp Lampen = The lamp
You can also make the word in plural by following the eg. Epler = Apples Eplene = The apples
Lamper = Lamps Lampene = The lamps
Use Google Translator to hear the pronounciation of each word.
It varies. Some select words have stem changes (eg mann → menn). Many others add -er. Neuter nouns stay the same. There are exceptions, of course, but this is generally the case.
It changes yet again in the definite plural. Masculine and feminine words add -ene, whereas neuter words add -a. Again, there are exceptions, but this is generally the rule.