We do not know what the phenomenon was that allowed the night to appear as though it were no night. I have suggested that a type of volcanic eruption might provide enough light to qualify. This would be significant, and coming as a sign would be natural, localized, and culturally significant. Of course we would also expect earth tremors with an eruption and they are not mentioned, so it may not be. What we should remember, however, is that regardless of what it was and how we might have seen it, the important thing is how the Nephites would have regarded the phenomenon. It is worth repeating the citation from the Florentine Codex that was cited in Helaman 14:
“Ten years before the Spaniards arrived here, an omen of evil first appeared in the heavens. It was like a tongue of fire, like a flame, like the light of dawn. It looked as if it were showering [sparks], as if it stood piercing the heavens. It was wide at the base, it was pointed at the head. To the very midst of the sky, to the very heart of the heavens it stood reaching; to the very midpoint of the skies it stood stretched as it was seen. It was there to the east when it thus came forth at midnight; it looked as if day had dawned, day had broken. Later, the sun destroyed it when he arose. For a full year [the sign came forth].” (Bernardino de Sahagun. Florentine Codex. Tr, Arthur J.O. Anderson and Charles E. Dibble. School of American Research and the University of Utah., 1975, Book 12, p. 2-3).
There is historical evidenced that people from that area saw signs in the heavens to which they ascribed a light at night that appeared as though it were day. This was taken as an important omen. Regardless of what might have caused the light, there is reason to believe that something could have caused such a light to be believed by the Mesoamericans, and the Nephites.