Combination of smoke from forest fires, a thick fog and cloud cover.
Outcome
Candles were required from noon on
1780 map of the region
New England's Dark Day occurred on May 19, 1780, when an unusual darkening of the daytime sky was observed over the New England states[1] and parts of eastern Canada.[2] The primary cause of the event is believed to have been a combination of smoke from forest fires,[3] a thick fog, and cloud cover. The darkness was so complete that candles were required from noon on. It did not disperse until the middle of the next night.[4][5]