The Day of Ascendancy was a prophecised event within the faith of the Bajoran Ascendancy.
According of the alternate timeline created by the Jalbador which generated the War of the Prophets, the Day of Ascendancy was a time that would come twenty five years after the supposed death of the USS Defiant and Deep Space 9. At this time according to certain Bajoran texts, the False Emissary would meet the True Emissary of the True Prophets at the time when the two wormholes opened in conjunction within one another at which point they would merge into the One True Temple.
Starfleet in this future timeline believed that if they could prevent the time displaced Captain Benjamin Sisko from meeting Kai Weyoun then the Day of Ascendancy would be delayed for millenia. This was because the merger would create a powerful shockwave that would destroy all of reality in the process. (DS9 novels: Millennium, The War of the Prophets)