Neon was discovered in May of 1898, so that would be the earliest possible
It was discovered by cooling air into liquid, then warming it slowly so the
various gases came out one by one and could be separated.
It was noted immediately that neon gave off a brilliant red light, so I gue
ss you could say that was the first neon light, though unintentional. I ha
ve no idea when commercial neon signs were available. Even at that time pe
ople had long been adept at drawing and shaping glass tubing, so signs woul
d have been possible soon.