[...] Impossible to leave you out now!—Dearest friend, if you can possibly manage to do so, come to my Tristan! It is to be performed on the 15th, 18th and 22nd May. If you come, spare my wife and do not tell her of it. Manage that somehow and—come! You will have an experience of which you can have no inkling. The performance will be simply wonderful. [...]
Source: Letters of Richard Wagner, Vol. 2, selected and ed. Wilhelm Altmann, trans. M. M. Bozman (J. M. Dent & Sons, 1927), No. 606