As Legends canon is accepted, the date goes back a very, very long time ago (in a galaxy far, far away).
Protosabers, mentioned in Nathan's answer, were certainly an early (circa 15,500 BBY, as he stated) design of lightsaber -- but they were based on an even earlier design.
The Rakata Empire created the Forcesabers, lightsabers essentially fueled by Dark Side Force energy. Their non-Rakata Force users ("Force Hounds") would construct their own Forcesabers and attune special focus gems to their own Force signatures, like the Jedi in later millenia. The Rakata themselves used forcesabers in a force pike configuration. However, the Forcesabers were created thousands of years before The Tho Yor Arrival (circa 36,453 BBY), which itself marked the beginning of the proliferation of Force usage.
The Jed'aii descendents of those taught in the Tho Yor pyramid ships would attempt to use Forcesabers only at great risk -- the Dark Side influence of the sabers made them highly corruptive.
This, in turn, spurred the development of the aforementioned Protosabers which, being powered by a power pack rather than Dark Side Force energy, did not corrupt the user.