The concept ϕαινόμενον (phainomenon) is not limited solely to the presence of things during the day. It is broader and designates anything showing itself of itself, whether it does so in daylight (Helle) or in darkness (Dunkelheit).
What, now, is darkness? For someone arguing in an empty-headed way, it is obviously not difficult to determine what it is. Daylight is διαϕανές, something that lets things be seen, darkness is an ἀδιαϕανές, something that does not. But darkness also lets something be seen. There are visible things that are visible only in the dark:
oὐ πάντα δὲ ὁρατὰ ἐν ϕωτί ἐστιν, ἀλλὰ μόνον ἑκάστου τὸ οἱκεῖον χρῶμα. ἔνια γὰρ ἐν μὲν τῷ ϕωτὶ οὐχ ὁρᾶται, ἐν δὲ τῷ σκότει ποιεῖ αἴσθησιν, οἷον τὰ πυρώδη ϕαινό μενα.
(Not everything is visible in light, but only the proper color of each thing. For some things are not seen in light but produce perception in the dark, such as things that appear fire-like.)
Darkness is something that, in a quite specific way, lets things be seen. In order to establish the dark’s difference from daylight , we must draw on a completely fundamental distinction of Aristotelian philosophy: the difference between ἐντελεχείᾳ (entelechia) (actual being) and δυνάαμει ὄν (potential being). Darkness is a δυνάμει ὄν, something utterly positive. Since, in our doctrine of categories, we have not developed such primordial categories, we are unable to comprehend this peculiar structure. Insofar as darkness is a manner of “being away,” it must be designated as στέρησις (steresis), as the absence of something that should actually be on hand. Darkness’ being consists in being potential daylight. It would be talking past Aristotle, if one were to say: “Daylight is what lets things be seen; thus, darkness is what does not.” The dark also lets things be seen. (GA17EN:36-37)