Twenty-seven years ago, a tradition was revived which refers to the year 1810. The Princess of Therese, born on July 8, 1792, was married to King Ludwig of Bavaria on October 12. The Theresienwiese - the venue of the Munich Oktoberfest - is named after this highly venerated royal wife from Hildburghausen. Therefore the Theresienfest always takes place at the same time as the Oktoberfest in Munich.
Text extract: http://theresienfest-hbn.de/geschichte/