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On April 14, 1561, the people of Nuremberg saw what seemed to have been an aerial battle between flying spheres, crosses, and cylinders, and the appearance of a black, very large, arrow-shaped object.
According to a broadsheet written by By Hans Glaser, letter-painter of Nurnberg:

“At daybreak, between 4 and 5 a.m., a dreadful apparition occurred on the sun, and then this was seen in Nuremberg in the city, before the gates and in the country – by many men and women"
“At first there appeared in the middle of the sun two blood-red semi-circular arcs, just like the moon in its last quarter.”
“And in the sun, above and below and on both sides, the color was blood, there stood a round ball of partly dull, partly black ferrous color.”
“Likewise there stood on both sides and as a torus about the sun such blood-red ones and other balls in large number, about three in a line and four in a square, also some alone.”
“In between these globes there were visible a few blood-red crosses, between which there were blood-red strips, becoming thicker to the rear and in the front malleable like the rods of reed-grass, which were intermingled, among them two big rods, one on the right, the other to the left, and within the small and big rods there were three, also four and more globes.”
“These all started to fight among themselves, so that the globes, which were first in the sun, flew out to the ones standing on both sides, thereafter, the globes standing outside the sun, in the small and large rods, flew into the sun.”
“Besides the globes flew back and forth among themselves and fought vehemently with each other for over an hour.”
“And when the conflict in and again out of the sun was most intense, they became fatigued to such an extent that they all, as said above, fell from the sun down upon the earth 'as if they all burned' and they then wasted away on the earth with immense smoke.”
“After all this there was something like a black spear, very long and thick, sighted; the shaft pointed to the east, the point pointed west.”
“Whatever such signs mean, God alone knows. Although we have seen, shortly one after another, many kinds of signs on the heaven, which are sent to us by the almighty God, to bring us to repentance…”
“… we still are, unfortunately, so ungrateful that we despise such high signs and miracles of God. Or we speak of them with ridicule and discard them to the wind, in order that God may send us a frightening punishment on account of our ungratefulness.”

today, on April 14, 463 years later, we still don't know...
So, what to make of this?

Some have said that the Nurembergers saw nothing more than a sundog. Such an explanation seems plausible (see the “blood red, semicircular arcs”), but also a bit simplistic, given everything they described.

modern interpretations:
In 1958, Carl Jung wrote Flying Saucers: A Modern Myth of Things Seen in the Skies In which he, too, suggested that it was a natural phenomenon, but as seen through religious and military mindset of that era.
According to author Jason Colavito, the woodcut broadsheet became known in modern culture after being published in Carl Jung's 1958 book Flying Saucers: A Modern Myth of Things Seen in the Skies, a book which analyzed the archetypal meaning of UFOs. Jung expressed a view that the spectacle was most likely a natural phenomenon with religious and military interpretations overlying it. "If the UFOs were living organisms, one would think of a swarm of insects rising with the sun, not to fight one another but to mate and celebrate the marriage flight."

A military interpretation would view the tubes as cannons and the spheres as cannonballs, emphasize the black spearhead at the bottom of the scene, and Glaser's own testimony that the globes fought vehemently until exhausted. A religious view would emphasize the crosses. Jung thinks the images of four globes coupled by lines suggested crossed marriage quaternities and forms the model for "the primitive cross cousin marriage". He also posited that it could also be an individuation symbol and that the association of sunrise suggests "the revelation of the light".

Lastly, in the summer of 1566, over a span of two weeks and 275 miles/440km away, the people of Basel, Switzerland witnessed what appears to have been the same kind of event, and even depicted it the same way.