Well... I don't have an encyclopedia, so you have access to more information than I do. That said if civilizations are more or less "reset" with each Umbral Calamity, then the new ones that arise following those events wouldn't have much knowledge about primals. For that knowledge to come about, logically people would have to have encountered them before Silvertear. As it pertains to this thread, though we don't have documentation of their first encounter Solus would logically have had to encounter a primal before or during the conquest of Othard to blame the Burn's aether-starved state on it.
I can't answer specifics; again, I don't have access to that much information, though from what I gather it might not even exist. Logically speaking though, for the civilizations of the Sixth Astral Era to know of and fear primals as they do they'd have to get knowledge of them from somewhere. One could say Sharlayan told everyone, but they remain isolationist outside of a few mavericks. One could argue that the Eorzeans did not encounter primals until after Silvertear, but that doesn't answer how the Garleans learned about them prior to or during their march on Othard.
Long story short... an exact date can't be pinned down with the information available to me, but logically speaking primals would have to have been encountered at least by the Garleans, sometime before or during the conquest of Othard, for the Burn to be blamed on them.