Fra Angelico's rendering (one of many) of the harrowing of Hell, when Christ broke down the gates of Hell (check out the squashed demon under the door) and led all those who had been born before Jesus' time out into the resurrection life.
Today we celebrate All Souls' or All Faithful Departed. Yesterday was focused on All Saints - the martyrs and prophets and big names. Today we commemorate everyone else who has died in the Lord. This day completes the "Fall Triduum" - the three days of All Hallow's Eve, All Saints' Day, and All Souls Day/Commemoration of All Faithful Departed.