Trenzalore

Trenzalore was a place that, according to Dorium Maldovar, the Doctor would visit at some point in his travels, which would coincide with 'the Fall of the Eleventh'. At this time and place "no living thing could speak falsely or fail to answer." Here, the First Question in the universe would be asked.

The Silence were determined that the Doctor never reach Trenzalore, so that the question would never be answered. It was a creed of the Silence that 'Silence will fall when the Question is aksed', or as Mandovar translated it, 'Silence must fall when the question is asked'. (DW: The Wedding of River Song)