At the times when they gather together at the city of Sais for their sacrifices, on a certain night they all kindle lamps many in number in the open air round about the houses; now the lamps are saucers full of salt and oil mixed, and the wick floats by itself on the surface, and this burns during the whole night; and to the festival is given the name Lychnocaia (the lighting of lamps). Moreover those of the Egyptians who have not come to this solemn assembly observe the night of the festival and themselves also light lamps all of them, and thus not in Sais alone are they lighted, but over all Egypt: and as to the reason why light and honour are allotted to this night, about this there is a sacred story told.
Herodotus describes how this festival is celebrated, but does not relay the "sacred story" of why. My personal guess is that this is a festival of Nit as the creator, as there are references to Her power to illuminate tied to Her status as Mother of the Gods and as Creator in surviving hymns and texts.
References: The Great Goddesses of Egypt, Barbara S. Lesko
An Account of Egypt, Herodotus, via Project Gutenberg