We rarely think of that line from the apostle's creed, but this would be the time, between the death on the cross and the resurrection.
We must start by remembering that prior to the resurrection of Christ, no one, good or evil, had access to that perfect union with God that we call heaven, the beatific vision. The the old testament vision of the world there was Sheol- in hebrew, hades-in Greek, the abode of the dead. He'll is seems can be a misleading translation because for most us that conjures up images of fire and pain. In fact, as we understand it, the experience of Sheol was different for different people.
Jesus descended to Sheol to free all the just ones who had gone before him. We can imagine the list beginning with Adam, and Eve, Abel, the first homicide victim, Abraham and Sarah, Moses, the prophets, and all the millions of unnamed souls who tried, as best they knew how, to live their life according to the will of God.
This classic resurrection icons show Jesus rising from the tomb holding these people by the hand, the broken chains of death lying on the ground. With the resurrection of Christ the gates of heaven, a place formerly reserved for God and angels, are opening to mere mortals like you and me.