The night is far gone; the day is at hand. So then let us cast off the works of darkness and put on the armor of light. (ESV)
All throughout the Bible there are these metaphors that help us to understand theological truths. In this verse we see "night" contrasted with "day." In the same verse we also see "darkness" and "light." The night is paralleled with darkness. The day is paralleled with light. Now, the bigger question would be, "What is Paul talking about here?"
The night is gone. The night is in the past. This is speaking of our former way of life when we followed Satan himself. We were rebels to God's Law. We lived to please ourselves, which was ultimately pleasing the enemy. A true Christian looks at the past as something to be delivered from, not something to be celebrated.
The day is at hand. We live now for the Lord. Our life should be filled with the light that comes from Christ himself. Jesus called himself the "light of the world. (John 8:12" Additionally, he told us in Matthew 5:16 that we are to let our light shine so that the world, who lives in darkness, would see our good works and glorify God.
I love how Paul does not just expect this transformation of darkness to light to happen without any action on our part. He tells us to cast off the works of darkness, and to put on the armor of light. We are at war with darkness. We must take action against the works of darkness. We must identify them and get rid of them. We need to equip ourselves with appropriate armor (Ephesians 6:10-18).
If you are still engaging in works that do not please the Lord you are walking in darkness. Cast off those works. Ask the Lord to fill you with his light, and do the things that Jesus, the Light of the World, has told you to do.