The first person narrator of "The Pit and the Pendulum" by Edgar Allan Poe is portrayed as a captive of the Spanish Inquisition. As is the case with many of Poe's narrators, he is not entirely sane, and at times we are unsure which aspects of his account are imaginary and which are intended as literally true. What we do know is that he has been accused of a crime by the Inquisition, is imprisoned, and is being tortured.
The reason that the narrator is sure that he will not be kept for a long time in the prison has to do with a public ceremony known as the auto-da-fé. This was a major spectacle conducted in large public squares by the Inquisition which began with a night long preparatory vigil of prayer followed by morning Mass. Next, the prisoners would be escorted into the square covered in yellow sackcloth. The verdicts of their secret trials would be read aloud, and then their sentences, such as whipping or burning at the stake, would be executed.
Poe's narrator thinks that since there was a shortage of prisoners for this ceremony, it was likely that he would participate shortly in the next auto-da-fé rather than being held indefinitely in prison.