What was Jesus doing on Saturday?

  6)  The Cross as Descending into Hell (1 Peter 3:18-20; Colossians 1:18)

18For Christ also suffered once for sins, the righteous for the unrighteous, to bring you to God. He was put to death in the body but made alive in the Spirit. 19After being made alive,  he went and made proclamation to the imprisoned spirits— 20to those who were disobedient long ago when God waited patiently in the days of Noah while the ark was being built. (1 Peter 3:18-20)

8And being found in appearance as a man, He humbled Himself and became obedient to death— even death on a cross. 9Therefore God exalted Him to the highest place and gave Him the name above all names, 10that at the name of Jesus every knee should bow, in heaven and on earth and under the earth,

11and every tongue confess that Jesus Christ is Lord, to the glory of God the Father. (Philippians 2:8-11) 

18And he is the head of the body, the church; he is the beginning and the firstborn from among the dead, so that in everything he might have the said, supremacy. 19For God was pleased to have all his fullness dwell in him, 20and through him to reconcile to himself all things, whether things on earth or things in heaven, by making peace through his blood, shed on the cross. (Colossians 1:18) 

7Now to each one of us grace has been given according to the measure of the gift of Christ. 8This is why it says: “When He ascended on high, He led captives away, and gave gifts to men.” 9What does “He ascended” mean, except that He also descendedc to the lower parts of the earth? 10He who descended is the very One who ascended above all the heavens, in order to fill all things. (Ephesians 4:7-10)

The death of Jesus Christ in each of the passages above is tied to the descent into hell (sometimes called "The Harrowing of Hell").  
Here is a bit of the thinking on the descent into hell: 
The idea of Christ's descent into hell is found in scripture (above) but also in the early church- Hipppolytus (c. 200AD)  said, "Therefore as many as Satan swallowed and bound, these the Lord, when he came, loosed from the bonds of death, having bound him who was strong against us, but having set humanity free." Origen (c. 220 AD) "First therefore he bound him at the cross, and thus he has entered his house, that is Hades (infernum), and from there "ascending on high, he led captivity captive."  By the fifth century it was seen as an important enough concept to add to the Apostles' Creed and confirmed in 750 AD.  But this is a bit of a strange concept.  What does it mean?  Here are two main ideas:
1) Christ went to hell after he died on the cross and proclaimed his death to those in hell.  Some say this meant A) the seal of those doomed there- like a proclamation (Luther/Hodges/1 Peter 4:6) and the conquering of the devil and evil; Some say it B) released the Old Testament saints who were in the realm of the dead until Christ came (Zwingli, Roman Catholic); Others that it C) emptied out hell entirely. 
The scriptures are a bit less specific here.  For those who think Christ emptied out hell, does that mean that he did that for eternity, or just for those who died before he descended.  It seems a stretch to think he eternally emptied out hell on Saturday- and nowhere in scripture or early tradition does it say this.  Certainly every knee can bow to the proclamation of Christ's death on the cross.  
2) Christ went through hell for us on the cross.  Hell is separation from God.  Jesus said, "My God, My God why have you forsaken me" (quoting Psalm 22).  He bore our sins in his death on the cross.  John Calvin interpreted his descent into hell in this way.  
3) Some say this is just meant to say that this is just a symbolic way of saying Jesus died and has victory over death and hell.  It is true that evil, death, the destruction of his crucifiers, hell and the devil are all conquered by the cross.  
4) Some say this just confirms the fact of his really being dead and buried.  So in the creed it says, "He was dead and buried, descended into hell."  While this is true, this alone misses the scriptural meanings above. 
5) Some traditions say he destroyed the power of hell and Satan/the devil.  (Acts of Pilate- 311 AD- "Satan, shaking and utterly terrified, spoke to his angels, "Bar the gates, for the Son of Man, God's champion, comes. He comes to lay waste to my Kingdom and devastate my realm. Do not let him pass for he shall take everything he can take, leaving us in ruin"."John 12:31; Col. 2:15

Whether you agree with Calvin that the death of Christ on the cross is "the descent into hell", certainly we can agree that the cross is where Christ bore our sins in his body on the tree (1 Peter 2:24).   He became the fulfillment of all the Old Testament sacrifices.  It is on the cross that the Lamb of God took away the sins of the world (John 1:29).  
The cross is a symbol, therefore of the taking away of our sins, and also the victory over evil (and the devil and hell).  The cross is a powerful symbol that good conquers over evil.  

Application: Our response to this should be gratitude and praise.  Christ went through hell for us so that we would not have to.  He paid our price with the infinite value of His death.  

Prayer: Thank you God for the cross of Christ- on which He paid my penalty of judgment and hell.  Thank you for your victory over death, destruction, evil, and hell that enables me to be free- in this life and the next.  

So what is my opinion?  My opinion is secondary to what the scripture says, and my desire is to submit my opinion to God (and thus God's Word).  In the 1950s the PCUS compiled a new (burgundy) hymnbook.  In the front it had the Apostles' Creed.  But because they couldn't understand "he descended into hell" and because the United Methodist Church omitted it and it was said to be a latter addition, they omitted it from the creed-without any authority from the church.  But just because we don't understand something doesn't mean we should omit it.  Part of saying the creed is to strive to believe what the church has agreed to believe.  
Jesus indeed did go through hell for us on the cross.  That is the most meaningful and universal application of this.  But also He truly died.  It seems that this is where God destroyed the power of the devil (He could not keep Jesus out), and preached his triumph to all those in hell already.  This is what it means when it says, "every knee shall bow in heaven, on earth and under the earth- and every tongue confess [even those in hell] that Jesus is Lord."   Hell is not destroyed here (see Rev. 20).  Nor are people in hell given a second chance.  The Reformation partly rebelled against the purgatory idea that is not found in canonical scripture.  We cannot pay, pray, or say our way out of hell for those who refused to be saved.   This life and our decisions in it are important and have to do with our eternal life.  We are given a chance up until the last breath.  Think about the thief on the cross who was told "Today you will be with me in paradise" by Christ.  It was not "Today you will descend into hell, I will meet you there and take you to heaven."  The ones who came before Jesus are not condemned to hell, or to sleep or to some quasi-hell.  Think about Lazarus and the rich man (Dives) in Luke 16. The rich man was carried to hell, the poor man to heaven.  This was before Jesus died, and he was giving a glimpse into how we are saved.  I understand those who say Jesus brought the Old Testament believers and other believers that lived before he came out of hell.  Certainly no one gets to heaven but by the grace of God found in the ultimate sacrifice of Jesus (which made all those Old Testament sacrifices real).  But Jesus doesn't need to physically descend into hell in order to do this.  When we die, our Maker decides where we go.  We will all stand before Him at the ultimate judgment to be declared innocent (by Christ's work, or guilty) at the end of time.  But Luke 16 implies that when we die we go to where we are destined.  Jesus descended into hell to declare He is Lord of heaven, earth, and under the earth.  No gates of hell can keep Him out.  He is God not just of 2/3 of existence.  But He is God of all.  It is not that the ruler is the ruler of all outside of prison and the prisoners rule the prisons.  Jesus is ruler of all.  His descent into hell proclaims Him Lord of all.  He is worthy of our worship.      

The Sabbath (7th day) was fulfilled when Jesus proclamation over death is completed.  Jesus was buried before the seventh day which was also the special holy day of Passover.  Jesus said himself that He is "Lord of the Sabbath."  "Sabbath" means "stop" or "rest."  It is a sign of peace- wholeness (shalom).  As God, He initiated the idea of Sabbath, but as the God-man, Jesus fulfilled the Sabbath rest.  Death is not the ultimate rest. The Sabbath is not the ultimate rest.  Christ's victory over death and hell and sin means ultimate peace with God, ultimate peace with others, and ultimate peace within ourselves.  Jesus' proclamation of victory in hell means there is nowhere that He is not Lord.  No evil can stop Him. No death can stop Him.  There is no more war- it is won.  While there are still fights going on, the surrender of hell has happened.  Jesus holds the keys to death and hell.  When we put our trust in Him, we can have peace.  We do not need to fear the power of evil.  We do not need to fear the power of death.  We do not need to fear the power of pain.  The Lamb of God who is slain has sacrificed Himself so we might have peace (rest) with God (Rev. 5:11-12- note "under the earth" is mentioned).  We know who wins in the end.  Thanks be to God who gives us the victory in Jesus Christ. He has descended onto earth, then descended into hell, and ascended into heaven.  He is God of all.  He is THE PRINCE OF PEACE.