Cause Unknown
At the time, some explained the darkness as being due to smoke from forest fires, others to the exceptional rise of vapors and atmospheric dust in the warm spring following the melting of unusually heavy winter snows. But forest fires were not of extraordinary occurrence in these regions, and many a springtime since has seen the melting of heavy winter snows and the rise of vapors; yet May 19, 1780, still stands unique in the annals of modern times as "the dark day." However observers and writers disagreed as to the nature of the mantle of darkness that was drawn over New England that day, they were one in recognizing the extraordinary character of the event.
The facts are fully covered by the statement in the dictionary, "The true cause of this remarkable phenomenon is not known."
What we do know is that the Saviour's prophecy declared, "Immediately after the tribulation of those days shall the sun be darkened, and the moon shall not give her light." And when the time for it came, the sign appeared.