For as Jonas was three days and three nights in the whale's belly; so shall the Son of Man be three days and three nights in the heart of the earth. ~Matthew 12:40
Upon death, the Lord Jesus separated from His body and descended into Hell (at the heart of the earth) for three days and three nights. Read Luke 16:19-31 and, if you look it up, you will find that the Greek word for Hell is "Hades." This is where the saved (Old Testament saints) and the lost (sinners) both went since the time of Abel (the first man who died) until the time of Christ's ascension to Heaven. (And, since the time of Christ's ascension Hell has been a place where all the lost souls go upon death, whereas the saved started to go home directly to Heaven upon death).
Hades was composed of Paradise or "Abraham's Bosom" (Luk. 16:22) for the saved (Old Testament saints), and Torment for the lost (Luk.16:23). A gigantic gulf separated the two places, so that one could not pass over to the other side (Luk. 16:26). This was located in the center of the Earth as Matt. 12:40 states.
Luke 23:43 tells us that this is where Christ went upon His death. "And the thief said unto Jesus, Lord, remember me when thou comest into thy kingdom."
The Lord answered him by saying, "...Today shalt thou be with me in Paradise."
Furthermore, the old Apostle's Creed (Traditional English Version) has these words, "...suffered under Pontius Pilate, was crucified, dead, and buried; he descended into HELL..." Jesus really went to Hell to taste death for every man (Heb. 2:9) and to confirmed to the Old Testament people (both to the saints and the lost) that He is indeed the promised Redeemer (Job 19:25; 1 Pet. 3:19). When Jesus arose from the dead He defeated Hell and Death and become the mighty Savior to everyone who believes in Him (Acts 2:27, 31; Heb. 2:14; John 5:24; Heb. 7:25).
At Christ's resurrection, the Bible tells us farther that "many bodies of the [Old Testament] saints" also resurrected and went out from their graves, walked again upon this earth (Matt. 27:52-53) and eventually went to Heaven together with all the saints as Jesus transferred them from Hades to Heaven (Acts 1:11; Eph. 4:8-10).
After the Lord's Resurrection, He remained on Earth for forty days, showing Himself in His resurrected body. When He ascended back to Heaven (Acts 1:11), He emptied the Paradise in Hades, taking all the saved (Old Testament saints) back with Him to Glory. Eph. 4:8-9 gives the record,
"Now that he (Christ) ascended, what is it but that he also descended first into the lower parts of the earth...Wherefore he saith, When he ascended up on high, he (Christ) lead captivity captive (those in Paradise to Heaven), and gave gifts unto men."
In 2Co. 12:2-4 we are told that Paradise is now in Heaven.
"I knew a man in Christ above fourteen years ago, (whether in the body, I cannot tell; or whether out of the body, I cannot tell: God knoweth;) such an one caught up to the third heaven. And I knew such a man, (whether in the body, or out of the body, I cannot tell: God knoweth;) How that he was caught up into paradise, and heard unspeakable words, which it is not lawful for a man to utter."
In addition, let me share the answer of this question, "Why the souls of Old Testament saints did not go directly to Heaven when they died" (John 3:13)? The answer is this: Heaven is a place for holy and perfect people. Even they were righteous but they still needed to be perfect in the sight of God. So, they must wait for God's perfect sin offering to thoroughly cleanse them from all their sins to make them perfect, holy and complete in God's sight which is only through the substitutionary death and atoning sacrifice of Jesus in Calvary (Heb. 10:10, 14). Every man who enters Heaven must be washed by the blood of Jesus (the Lamb of God) and must be clothed with His righteousness (Col. 1:22; Rev. 1:5; Rom. 10:4) to become holy, perfect and complete in God's sight.
Today, when a Christian dies, they go immediately to Paradise to be with the Lord in Heaven. As apostle Paul said, "we are confident, I say, and willing rather to be absent from the body, and to be present with the Lord." (2Co. 5:8). At death, the Christian only changes his location. Therefore, leaving his body behind, he (his soul and spirit—the real "you") enters the glories of Heaven! Indeed, for true Christians, to live is Christ, and to die is gain (Phil. 1:21). Hallelujah!