The book of Amos is a scathing rebuke against all unrighteousness.  God is calling His people to remember who He is and repent, throwing themselves on His mercy.  In the middle of chapter five, the Lord turns Amos’s attention to “The Day of the Lord”.  The Day of the Lord is a common theme throughout the Old and New Testaments. It is described as the time when God says enough is enough; the time for final judgment has arrived.   The Son of Man will return to the earth.  This time, He will not be coming as the Suffering Servant, in lowliness and humility allowing Himself to be mocked and spit upon.  Rather, this time, He will come in power and glory and wrath and judgment.   He will usher in the Kingdom of God in its fullness.  All attacks upon it will come to an end.    Christ will rule and reign in absolute power and authority.  Even though believers will face some judgment, for them the Day of the Lord will ultimately be a day of joy and victory.  Horrifyingly, to unbelievers the Day of the Lord will be a day of judgment, punishment, and terror that will never end. 
 
This reference to the Day of the Lord in Amos 5:18-20 is probably the earliest one in the Bible.   God is very displeased because His people are living very unrighteous lives.   Evidently some of the people believed that just because they were members of the congregation of Israel that the Day of the Lord will be a wonderful event for them.  They believed that the Day of the Lord would be a great deliverance for the Jews and a terrible judgment for the Gentiles.  God could not make it any clearer that because of their unrighteous living, it will not be wonderful for them.  He told them, it will be a day of total darkness without even a ray of light. Their membership in the nation of Israel will not help them.  It will be a day of running but no escape.  God uses an almost comical word picture to stress the inescapable doom that awaits them.  It reminds me of the old-time cartoons.  A man ran into a bear while running away from a lion; he ran from the bear and just when he thinks he has escaped by running into his house, he leans against the wall and is bitten by a snake.   God says in very vivid terms, you think this will be a good day: it will not be a good day.  Punishment is reserved for you and there will be no escape.  
 
God makes His point with a question in Amos 5:20,  Will not the day of the Lord be darkness instead of light, Even gloom with no brightness in it? (NASU) Is there a message here for us today?  Surely there is.  The coming of the Day of the Lord is just as certain as it was then, only nearer.   Certainly, many today are living in unrighteousness just as in the days of Amos.   Surely many believe that membership in their church will exempt them from judgment.  It will not be.  Each of us will face judgment from God, believers and unbelievers both.  I do not know exactly what the judgment for believers will entail or what the punishment will be. Perhaps it will only be loss of reward.  However, I do know this:  For unbelievers, those who have not thrown themselves on the mercy of God through Christ Jesus, face a tortured eternity, forever separated from a loving God.   It will be far worse than facing lions and bears and snakes.  Turn to Christ asking to be forgiven based on His substitutionary atoning death on the cross.  That is your only hope.