In that day, the Inevitable Event shall occur

In that day: that is, at that time, given that the intended meaning of "day" is absolute time. There is a broad scope here in which whatever is to occur shall occur, and the tanwin (nunation) is in place of the genitive construct—meaning: in the day when the Trumpet is blown, and such and such happens.

The Inevitable Event shall occur: that is, the Resurrection shall be established. Interpreting "the Inevitable Event" as the rock of Jerusalem is far from the level of acceptance.