Date of establishment 

Charles MacKay says that a people called the Frieslanders were holding an annual diet under an oak tree at Aurich as early as the 11th century, quote: "As early as the eleventh century they had formed a general confederacy against the encroachments of the Normans and the Saxons, which was divided into seven seelands, holding annually a diet under a large oak-tree at Aurich, near the Upstalboom." The Church exterminated them for witchcraft, almost wiping them from the pages of history. Almost. ref - http://www.gutenberg.org/files/24518/24518-h/dvii.html 49.3.2.175