"The day" usually refers to the Last Day, the day of Christ's Return and of Final Judgement in St Paul's letters. In today's Epistle he warns Christians they must live in holy love, since the completion of their salvation is getting closer. For the whole of Advent we meditate upon the Four Last Things: Death and Judgement, Heaven and Hell.
Some in the Church are ashamed of at least half of these. They are often happy to talk of Heaven, and may even be willing to remind people of the inevitability of Death, though more as a somewhat sad departure than as a danger. For Death can only be feared as well as a grieved if Judgement and Hell are also part of the equation.
But the dark shadow of coming Justice has been fundamental to the Gospel, the Good News, from the beginning. “And that repentance and remission of sins should be preached in his name among all nations, beginning at Jerusalem” Luke 24:47, “Now when they heard this, they were pricked in their heart, and said unto Peter and to the rest of the apostles, Men and brethren, what shall we do? Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost". Acts 2:37-38. "And he commanded us to preach unto the people, and to testify that it is he which was ordained of God to be the Judge of quick and dead. To him give all the prophets witness, that through his name whosoever believeth in him shall receive remission of sins". Acts 10:42-43. "And after certain days, when Felix came with his wife Drusilla, which was a Jewess, he sent for Paul, and heard him concerning the faith in Christ. And as he reasoned of righteousness, temperance, and judgment to come, Felix trembled, and answered, Go thy way for this time; when I have a convenient season, I will call for thee". Acts 24:24-25. This does not mean that early preaching was mostly focused on threatening themes of God's wrath. As we just heard, repentance goes with remission, justice with mercy. [Compare with "At that time Jesus answered and said, I thank thee, O Father, Lord of heaven and earth, because thou hast hid these things from the wise and prudent, and hast revealed them unto babes. Even so, Father: for so it seemed good in thy sight. All things are delivered unto me of my Father: and no man knoweth the Son, but the Father; neither knoweth any man the Father, save the Son, and he to whomsoever the Son will reveal him. Come unto me, all ye that labour and are heavy laden, and I will give you rest. Take my yoke upon you, and learn of me; for I am meek and lowly in heart: and ye shall find rest unto your souls. For my yoke is easy, and my burden is light". Matthew 11:25-30 and "And Cornelius said, Four days ago I was fasting until this hour; and at the ninth hour I prayed in my house, and, behold, a man stood before me in bright clothing, And said, Cornelius, thy prayer is heard, and thine alms are had in remembrance in the sight of God. Send therefore to Joppa, and call hither Simon, whose surname is Peter; he is lodged in the house of one Simon a tanner by the sea side: who, when he cometh, shall speak unto thee. Immediately therefore I sent to thee; and thou hast well done that thou art come. Now therefore are we all here present before God, to hear all things that are commanded thee of God. Then Peter opened his mouth, and said, Of a truth I perceive that God is no respecter of persons: But in every nation he that feareth him, and worketh righteousness, is accepted with him. The word which God sent unto the children of Israel, preaching peace by Jesus Christ: (he is Lord of all:) That word, I say, ye know, which was published throughout all Judaea, and began from Galilee, after the baptism which John preached; How God anointed Jesus of Nazareth with the Holy Ghost and with power: who went about doing good, and healing all that were oppressed of the devil; for God was with him".Acts 10:30-38 (e.g., Christ "preaching peace").] Judgement is not portrayed as an angry threat, but an undeniable inevitability, but one which could be emptied of its terrors through Christ.