I've been searching for content to put in my family newsletter, so I Googled <"dark day" "new england" 1780 > and quickly found a page from Weather Almanac here.
The "Dark Day in 1780" was a result of forest fires, probably in the woods of New Hampshire or Maine. The effects of the fire were felt over a wide area:
"As the extraordinary darkness descended, those observing it wondered if it was a local phenomenon or more widespread. With no rapid communication networks in place, the only way to determine the extent of the darkening was through letters and newspapers from across the region. After the event, Professor Samuel Williams of Harvard College undertook the collection of such information from across the region. He was able to determine that notice of the darkness was taken as far south as northern New Jersey and New York City coastal waters, as far north as Portland, Maine and west into the Hudson Valley, but no mention came from Philadelphia or outlying Pennsylvania. (The New Jersey report came from George Washington's diary at Morristown.) Williams concluded the dark centre of the event was located around northeastern Massachusetts, southern New Hampshire and southwestern Maine."
Read the whole article - it's a good compilation from a number of sources.
So now I know. And my family members will also know (assuming they read it!).