In his foreboding 1921 poem "The Second Coming," Yeats theorizes that a gentler era of history, ushered in by the birth of Jesus, is coming to an end. Instead of the second coming of Christ, however, which Christians traditionally see as the "end" of history, Yeats envisions a "rough beast" being born in Jesus's birthplace of Bethlehem. This birth will start a new, more violent spiral of history.

The lines referred to, which end in the "ceremony of innocence," are as follows:

Mere anarchy is loosed upon the world,

The blood-dimmed tide is loosed, and everywhere

The ceremony of innocence is drowned.

I moved this to the Yeats poem because the image is derived from his work.  In "The Second Coming," Yeats discusses what he feels best describes the modern setting with a series of images that reflect a sense of crisis and decline.  One of these images concerns the "ceremony of innocence:"

The blood-dimmed tide is loosed, and everywhere
The ceremony of innocence is drowned;

For Yeats, this image is critical in addressing the condition of disintegration that Yeats sees all around him following the First World War.  The "ceremony of innocence" refers to a practice of purity, of a rite of passage, and of something that represents a structure and order.  Ceremonies of innocence are those that are rooted in a transcendent condition that provides meaning.  It is for this reason that madness and disorder have "drowned" the "ceremony of innocence." The idea of "drowning" helps to bring out that disorder and chaos have enveloped the hope of structure, the "ceremony of innocence."