In "Anthem for Doomed Youth," what sort of religious ceremony is taking place, and what details from the poem support your thoughts?

I'd call it a religious rite rather than a religious ceremony. Although written in the sonnet form, this poem is an elegy, or lament for the dead. Wilfred Owen wrote the poem out of his own experience of World War I. That was has been called "the war to end all wars"--and if only that were true. Before WWI, warfare seemed to follow a kind of structure, almost like a football game with each side lined up against the other. WWI introduced new horrors, with armored weapons, air raids, and chemical warfare.

The horrors of war dominate the first eight lines. We see men "die as cattle," in other words slaughtered. We hear the guns and bombs. Owen says that no church bells will toll for their funerals; instead there will be the bugle call. The only choirs that will accompany them to the afterlife are the "shrill, demented choirs of wailing shells." 

In the final six lines we experience the distorted "funeral" that awaits these dead soldiers. No candles will be placed around their coffins. The only "window blinds" that will be drawn for them will be nightfall.

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