He took other assignments during his tenure in Berlin. He was guest conductor at the Silesian music festivals in Goerlitz between 1894 and 1911. In May and June 1899 at London's Royal Opera House Covent Garden, he conducted Beethoven's "Fidelio" and several of Wagner's operas ("Tannhäuser", "Die Walküre", "Die Meistersinger", "Der fliegende Höllander" and "Tristan und Isolde"). He devoted many summers to the Wagner Festival in Bayreuth where he became principal conductor in 1903, after serving as a musical assistant since 1892. He succeeded Hermann Levi as the conductor of "Parsifal" there. As war approached in the summer of 1914, Muck insisted on performing "Parsifal" on August 1, 1914 to close the Festival, which was not revived until 1924. Muck conducted "Parsifal" at all of the fourteen Bayreuth festivals held between 1901 and 1930, and also conducted "Lohengrin" there in 1909 and "Die Meistersinger" in 1925, becoming a close friend of the Wagner family. The American music critic Herbert Peyser (1886-1953) thought Muck's interpretation of "Parsifal" the greatest he had ever heard: "the only and ultimate "Parsifal"; the "Parsifal" in which every phrase was charged with infinities; the "Parsifal" which was neither of this age nor that age but of all time." He led the Vienna Philharmonic from 1903 to 1906 and the Boston Symphony Orchestra from 1906 to 1918, and took visiting assignments in other cities, including Paris, Madrid, Copenhagen, Brussels.