On this day Christ is in the tomb. There is no daytime Mass on Holy Saturday. It is still a day of fasting and sorrow before the Easter Vigil begins in the evening.
We recall, with Mary and the disciples, that Jesus died and was separated from them for the first time as He lay in the tomb. The Good Friday fast is often through Holy Saturday.
In the Apostles Creed we pray "He descended into hell" (translated hades, that is, the temporary abode of the dead—not the eternal lake of fire) which describes what Jesus did in the time between his burial and Resurrection. Jesus descended to the realm of the dead on Holy Saturday to save the righteous souls—the Old Testament patriarchs, for example—who died before his crucifixion.
The Catechism of the Catholic Church calls Jesus' descent into the realm of the dead "the last phase of Jesus' messianic mission," during which he "opened heaven's gates for the just who had gone before him." Before Holy Saturday, there were no souls enjoying the beatific vision of God in heaven.
Christ's work on Holy Saturday is also known as the "Harrowing of Hell."