Exactly when, down to the day, will the mystery be brought to light?  Here are two answers:

"Possibly, in some yet undiscovered ruin or tomb, the key may be found to the problem which now puzzles the world: but then it is only a possibility.  There is little doubt that the mystery will remain a mystery until the great day when the sea shall give up its dead and the past be stretched before us like a scroll." —The True Latter-Day-Saints' Herald, 1873

"That, I suppose, will remain a mystery till the day when a[ll] secrets will be cleared up, an[d] a[ll] the deeds o[f] darkness brought to light." —The Brownie of Bodsbeck by James Hogg, 1833