There is nothing in Scripture that says anything about what was going on while Jesus was in the tomb. We know from the Apostle's Creed that Jesus "descended into hell." As the image above (EWTN UK) depicts, it is presumed that Jesus went to the netherworld to lead the souls into heaven. Their sins are forgiven; their debt is paid.
Because of lack any known actions occurring during this time, we often see Holy Saturday as a "day off" between the crucifixion and death on Good Friday and resurrection and life on Easter Sunday. But what really happened during Good Friday? In her Hallow reflection on 2 April, Sister Miriam James Hyland said this with regard to Holy Saturday:
"Let’s begin by imagining what Holy Saturday must have felt like for the disciples, and Mary Magdalene especially, as the reality of the crucifixion starts to sink in. Perhaps you’ve been where the disciples are right now—in those moments after the worst thing has happened. In the aftermath of your worst fear coming true. If you’ve been there, then you know the surge of adrenaline that hits in this moment, when the tragedy is still unfolding. We can imagine that Good Friday, while filled with anguish and grief, was at least filled with things to do. Because sometimes, that’s easier. Perhaps you can picture what it would have been like to be carried along by the events of that Friday afternoon, waiting for the news or following the crowds or keeping vigil at the foot of the cross. But in the aftermath, there’s just stillness. Silence. Holy Saturday offers no busy-ness to distract us from the crushing weight of grief, and the overwhelming silence of Christ’s death."
We will spend Holy Saturday engaged in Imaginative Prayer as Sister Miriam describes above. We do not know that the apostles did, but we can use our imaginations to discuss and pray what "likely" occurred. We can use some Jewish religious traditions such as Shiva, which was conducted during Jesus's time. Shiva is a gathering of friends and family to pray on behalf of the deceased. It is traditionally a seven-day gathering. An example of Shiva in the New Testament is when Jesus and His disciples came to Lazarus's house after his death and was approached by Martha. We can "imagine" that all were gathered for Shiva.
So again, our question is, "What were the Apostles doing while Jesus was entombed?"
All are encouraged to download some files for your review. The first is Sister Miriam's Hallow reflection that is referenced above. There is also some scriptural references for the Luminous Mysteries of the Rosary. We will use the Luminous Mysteries - events from Jesus's public ministry - as references as to what the Apostles may have discussed among themselves to reflect on their time with Jesus. The third document is an introduction on Imaginative Prayer. Saint Ignatius of Loyola is credited with being the catalyst "Pray with your imagination," and this paper is by David Fleming, SJ from the Ignation Spirituality web site.