Day of the Lord
One thing to remember when addressing this question is a simple idea that light is destructive to darkness. Consider these passages:
1 Thessalonians 5:2
2 For you yourselves know perfectly that the day of the Lord so comes as a thief in the night.
In the expression “day of the Lord” emphasize the word “day”, and in the “thief in the night” emphasize “night”. Now, read it outloud with these emphases. See how it reads differently? It’s about day vs night. “Day of the Lord” is to be taken to mean “day originating / deriving from the Lord”, i.e. genitive of source. The Lord comes in the night – meaning, in the midst of darkness. His mode of arrival is “as a thief” – i.e., unexpected, with the additional semantics of “inflicting loss”. The primary characteristic of his coming is “day”, or “light/brightness” (as opposed to “night/darkness”).