Oktoberfest started Oct 17, 1810, as a wedding party. But it existed long before...
Oktoberfest is the most recognized of Germany's festivals and lauded as the World's Largest Folk Fest...6.8M+ attendees in 2019.
Originally, it was the "city fair" for Munich, the capital of the Kingdom of Bavaria, which then was not part of Germany, nor did it even consider itself Germanic...it was Bavaria...Land der Bayern.
This sense of "unique" culture continues even today.
Bavaria is Germany's largest state by land mass, and is second largest by population - much like Texas or California is in the United States. Other Germans and Bavarians disagree on many things, but they all agree on this—Bavarians are Bavarians. Sort of like Texans are Texans and Californians are Californians.
On October 12, 1810, Prince Ludwig I of Bavaria (grandfather of the castle-building Ludwig II) married Princess Theresa in their own version of a "Charles and Diana" Wedding. "Everyone" was invited and the then-local Munich "city fair" was taken over for the Royal Event. To commemorate this, every 4 years, a section of the Oktoberfest is set aside for the Bavarian State Agriculture Expo, harkening back to its roots.
Maximillian I, then the King of Bavaria, and father to Ludwig I, issued royal charters to 6 local Munich breweries to produce beer for the wedding party. Augustiner, Hacker-Pschorr, Löwenbräu, Paulaner, Spaten, and Hofbräu-München remain today the only breweries sanctioned to produce the official beer for the Oktoberfest and to market it as Oktoberfestbier. With time and merchandising, other breweries have now their own Oktoberfestbier, but they are not the original.
But, if the Oktoberfest is the Oktoberfest, why do some people call it the Wies'n?
It's a little confusing. The festival area, which was originally outside the city gates, was renamed the Theresienwiese in honor of Princess Theresa, and that name has stuck, so locals just call it "d 'Wies'n."
In 2019, 6M+ people attended the Oktoberfest in München.
Due to the Covid-19 pandemic, the official Oktoberfest was canceled in 2020 and 2021.
However, in 2021, many local groups banded together, held smaller fests, and appended the name -Wies'n to these (for example: Wirtswies'n). This tradition seems to have caught on so you will see many events called -Wies'n. While not the official Oktoberfest, they are filled with just as much Gemütlichkeit.