What caused the "Dark Day" — on May 19, 1780?

It was not a solar eclipse.

With little scientific knowledge amongst the populace in 1780, people would have been afraid. Some lawmakers in Connecticut believed it was the day of judgement. The sense that a decisive moment was afoot would have been bolstered by the fact that during the preceding days, the sun and moon glowed red.

Volcanic dust is one theory, but there's no record of a volcanic eruption then. It could have been a meteorite. It was actually probably a forest fire.