Among the milestones of alien sightings, we have the first flying saucer report in 1947 and the first alien abduction report in 1961. But people have been seeing unexplained phenomena in the sky throughout history. In 1909, that included a lot of New Englanders at the same time.
"It all began on 12 December, when prominent Worcester businessman Wallace Tillinghast told a Boston Herald reporter he had invented the world's first reliable heavier-than-air flying machine," Stephen Whalen and Robert E. Bartholomew recounted in the New England Quarterly.
Over the ensuing weeks, airship sightings flooded in, which newspapers gleefully reported with little skepticism. The reports in turn prompted even more sightings. It was a chain reaction of delusions.
"The great airship episode peaked in a frenzy on Christmas Eve," Whalen and Bartholomew described. "On that night there were thirty-three separate reports, spreading from Massachusetts southward to Rhode Island and Connecticut, northward to Vermont and Maine, and as far west as New York. In Boston, 'thousands upon thousands of people... stood on sidewalks, street corners and squares... hoping for a glimpse of the flying machine.'"
Not exactly an alien invasion, as people assumed they were seeing an aircraft from earth. But it was still a delusion. The reasons behind it are explained at Real Clear Science.