Abarim Publications' online Biblical Hebrew Dictionary
לילי
The masculine nouns לילה (layela) and ליל (layil) mean night; the opposite of יום ( yom), meaning day. The root of our two words לילה (layela) and ליל (layil) is missing in action, but BDB Theological Dictionary submits, "meaning of the root is dubious but the form is probably לילי (laylay)." Whatever that meaning might have been, it's safe to say that it was probably not very positive.
The Bible is very clear that both night and day were created and are owned by God (Genesis 1:5, Psalm 74:16, "Yours is the day; Yours is also the night") and God often uses the darkness to bring about great things (Genesis 1:2, 15:12, Exodus 12:29, Matthew 27:45). But in the same metaphorical (or self-similar) system in which Jesus is the light (John 8:12), the darkness counts as the absence of Christ, and that is not good. The Bible always speaks of the "day" of the Lord, and never of the "night" of the Lord. Or as Jesus himself says, "We must work the works of him who sent me, as long as it is day; night is coming when no man can work" - John 9:4.