As Gallifreyan mentions in his comment , to "pull down the blinds" means to draw the window shades for privacy. In his youth, Yeats was a member of the Rosicrucians, an esoteric society whose beliefs blended Christianity with traditions such as alchemy and Hermeticism. The celebration here is a Rosicrucian gathering, and the blinds need to be drawn to preserve the secrecy of the mystic order.
This poem draws upon Rosicrucian beliefs to make a point about contemporary artistic and philosophical endeavor. One such belief is that the body of the legendary founder of Rosicrucianism, Christian Rosenkreuz, remains undecayed. Yeats's 1895 essay "The Body of the Father Christian Rosencrux" in its entirety reads as follows:
The followers of the Father Christian Rosencrux, says the old tradition, wrapped his imperishable body in noble raiment and laid it under the house of their order, in a tomb containing the symbols of all things in heaven and earth, and in the waters under the earth, and set about him inextinguishable magical lamps, which burnt on generation after generation, until other students of the order came upon the tomb by chance. It seems to me that the imagination has had no very different history during the last two hundred years, but has been laid in a great tomb of criticism, and had set over it inextinguishable magical lamps of wisdom and romance, and has been altogether so nobly housed and apparelled that we have forgotten that its wizard lips are closed, or but opened for the complaining of some melancholy and ghostly voice. The ancients and the Elizabethans abandoned themselves to imagination as a woman abandons herself to love, and created great beings who made the people of this world seem but shadows, and great passions which made our loves and hatreds appear but ephemeral and trivial phantasies; but now it is not the great persons, or the great passions we imagine, which absorb us, for the persons and passions in our poems are mainly reflections our mirror has caught from older poems or from the life about us, but the wise comments we make upon them, the criticism of life we wring from their fortunes. Arthur and his Court are nothing, but the many-coloured lights that play about them are as beautiful as the lights from cathedral windows; Pompilia and Guido are but little, while the ever-recurring meditations and expositions which climax in the mouth of the Pope are among the wisest of the Christian age. I cannot get it out of my mind that this age of criticism is about to pass, and an age of imagination, of emotion, of moods, of revelation, about to come in its place; for certainly belief in a supersensual world is at hand again; and when the notion that we are 'phantoms of the earth and water' has gone down the wind, we will trust our own being and all it desires to invent; and when the external world is no more the standard of reality, we will learn again that the great Passions are angels of God, and that to embody them 'uncurbed in their eternal glory,' even in their labour for the ending of man's peace and prosperity, is more than to comment, however wisely, upon the tendencies of our time, or to express the socialistic, or humanitarian, or other forces of our time, or even 'to sum up' our time, as the phrase is; for Art is a revelation, and not a criticism, and the life of the artist is in the old saying, 'The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh and whither it goeth; so is every one that is born of the spirit.'
Yeats, William Butler. "The Body of the Father Christian Rosencrux". 1895. Ideas of Good and Evil, 1903. Retrieved from wikisource 22 October 2023.
"The Mountain Tomb" says in poetry what this essay says in prose. In the essay, Yeats says that contemporary artistic and/or mystical works are merely "ephemeral and trivial phantasies" compared to those of ancient or Elizabethan times. In the poem, he likewise says that self-styled seers may rejoice at having found Father Rosencrux/Rosicross's body, but all the music and wine are "in vain", as Father Rosicross still "sleeps in his tomb". The works together claim that celebrations of contemporary achievements are futile, because "the wizard lips are closed", and "all wisdom" is still "shut" in the "onyx eyes" of the "inextinguishable magic lamps" that light Father Rosicross's tomb.
Yeats fit uneasily into the mystic societies of which he was a member. He had been asked to leave the Theosophical Society in 1890, and resigned from the Golden Dawn in 1901. The essay and the poem show his skepticism that those societies had any enduring insights. He dismisses their efforts, and those of contemporary writers, as mere commentaries on past achievements rather than original and vivid productions.