>>3620092It could mean to literally "stay night", which is to say, "to keep darkness (the night) away."
The reality is that the Grail brings about an apocalypse, which the protagonists discover shortly before the climax of the story. Their goal shifts from winning the Grail War to destroying the Grail in order to keep said apocalypse from occurring.
The reverse, I suppose, could also be true...where the antagonists seek to get "night" (the new world order they desire) to "stay".
Or maybe I'm reading too much in to it.
