Brightest night
From Wings of Fire Fandom Mirror
(You could also be looking for the book. This article refers to the actual night.)
The brightest night is when the Prophecy dragonets were born. It is when, every one thousand years or so, all three moons are full.
History
Darkstalker was born on the brightest night, as was supposed to be Whiteout. The Dragonets of Destiny were also born on the brightest night.
Next Brightest Night
It seems that there has been a brightest night DURING the first arc, but that is confirmed to be false. That “fake” brightest night was really a comet or asteroid or something in space that is bright, looking like a moon. It will probably be in the next about 999,999 thousand years after TBN.