Why is the Oktoberfest also called "Wiesn"?
The first Oktoberfest took place in 1810 as part of the celebrations of the wedding between crown prince Ludwig, the later King Ludwig I. of Bavaria, and Therese of Sachsen-Hildburghausen.
On October 17 the five-day celebrations ended with a public horse race at a meadow between Munich and Sendling.
Due to the great success of this great conclusion of the “October Fests” (October-Feste), as they said initially, the formerly nameless meadow below the Sendling hill became the name “Theresens Wiese” (Therese’s Meadow) in honor of the bride.
After 1815 this name was simplified as “Theresienwiese”. As it initially only served the Oktoberfest, besides some agricultural cultivation, the name of the event and its location merged.
After all, when someone from Munich says in September that he’s going to the Theresienwiese, you can be certain that he’s visiting the Oktoberfest.
As people from Munich like merging syllables just as they like to merge names, sometime later, they would simply go to the “Wiesn”. Hence, in Munich you literally say, “I’m going out to the meadow” (naus auf d’Wiesn), when you visit the Oktoberfest.