Genesis 1:3 “וַיַּבְדֵּל אֱלֹהִים, בֵּין הָאוֹר וּבֵין הַחֹשֶׁךְ”, “and God separated the light from the darkness”
The secret of this verse is in Genesis 1:5 “וַיִּקְרָא אֱלֹהִים לָאוֹר יוֹם, וְלַחֹשֶׁךְ קָרָא לָיְלָה; וַיְהִי-עֶרֶב וַיְהִי-בֹקֶר, יוֹם אֶחָד “ “And God called the light Day, and the darkness He called Night. And there was evening and there was morning, one day.”
Light and darkness were separated on the first day but for the purpose of creating a world of Tikkun, for us to work in bringing them together as one. Morning is Abraham, right column and light of Chessed. Evening is Isaac, left column and Gevurah.
“One day” is the aspect of the central column that represents the unity of the right and the left.
In our reality we can’t have day without night. Our work is to bring the light of the day into the night, darkness, and make them “One day”, which is continuous presence of the Light. When we do so we create a key to the door that separates above and below and lets the Shechinah, the Queen of Malchut, gain access to the King above.