Poe's inside joke.
It centers around St. Expedite, the patron saint for those who wish to avoid procrastination, and obtain general financial success. Expedite is usually depicted as a Roman centurion crushing a crow beneath his foot. The dying crow is shown as saying "Cras," the Latin word for "tomorrow." In other words, Expedite vanquishes tomorrow in favor of today.
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"Cras" (which was later anglicized to "Caw") was a Roman pun, as it also stood for the sounds made by crows and ravens. Thus, these birds were seen as speaking of nothing but "tomorrow."
...The narrator of "The Raven" obsessively asks his feathered visitor when he will be reunited with his beloved, expecting to hear "Cras"--tomorrow. Instead, to his discomfiture, he gets the answer, "Nevermore."...