So where did Christ's soul go between his death on the Cross and His Resurrection? The Creed tells us: He descended into Hell.
Not that part of Hell that is the irredeemable abode of Satan; but rather the place where the just who died before his coming were awaiting the opening of Heaven to humanity, which would be accomplished by His Death, Resurrection and Ascension.
Why he went there was to preach to the dead (as St Peter tells us [1 Peter, 3.20 and 4:6]) - to Evangelise them, that is bring them the Good News that their time of waiting was nearly at an end.
Traditionally, this has been called the Harrowing of Hell.
For more on the Harrowing of Hell, here is a link to the Catholic Encyclopedia. (and see Update, below).
In the meantime, here are some wonderful medieval images of the Harrowing of Hell...
For more pictures of the Harrowing of Hell, try a google search of images for this term. There are many, and fascinating!
UPDATE: And there is this beautiful reading, the 2nd Reading from Office of Readings (Matins) from an ancient Homily. (H/T @TJTM_25 on Twitter)