In The Book of Hours in 2007, Kevin Jackson described this word as “rare and archaic”, but also as “the precise or pedantic word for the gloom before dawn”. There you have it in a nutshell.
Rare it certainly is, though a few well-known authors have taken advantage of its precision and its unusualness, among them Thomas Carlyle, Thomas De Quincey, Henry Wadsworth Longfellow and James Joyce. One more:
Hardly anything could be more isolated or more self-contained than the lives of these two walking here in the lonely antelucan hour, when gray shades, material and mental, are so very gray.
The Woodlanders, by Thomas Hardy, 1887.
The word derives from the Latin root luc-, meaning light, linked to the noun lux, light. Put -an on the end to turn it into an adjective and ante- in front to mark it as referring to something beforehand, and it becomes a term for the moments before the coming of the light.
The first time that I encountered the word, in The Uplift War, an SF classic by David Brin, I was momentarily derailed from his narrative by being reminded of a famous missing-persons case in Britain, that of Lord Lucan. What came before Lucan? Presumably another Lucan. It might have been the one who sent Lord Cardigan and his troops on the ill-fated Charge of the Light Brigade. But that was the wrong kind of light.