The day purposely follows All Saint's Day in order to shift the focus from those in heaven to those in purgatory. It is celebrated with masses and festivities in honor of the dead. While the Feast of All Saints is a day to remember the glories of Heaven and those there, the Feast of All Souls reminds us of our obligations to live holy lives and that there will be purification of the souls of those destined for Heaven.
Many people decorate their home altars for this Feast by putting up pictures of deceased relatives, lighting candles for them, burning incense, or setting up certain flowers, and decorative coloring (Black).
I'm going to decorate our home altar tomorrow, and try to get some pictures of it up.
"The theological basis for the feast is the doctrine that the souls which, on departing from the body, are not perfectly cleansed from venial sins, or have not fully atoned for past transgressions, are debarred from the Beatific Vision, and that the faithful on earth can help them by prayers, alms deeds and especially by the sacrifice of the Mass" (Catholic Encyclopedia).
At All Soul's Day Mass the Priest (Sub-deacon and Deacon) will be wearing black vestments that are representative of fundamental Christian realities. While Christians are a people of hope, we are also a people aware of the reality of sin and God's judgement. It also is a reminder and symbol of our belief in purgatory where suffering souls require our prayers and especially Masses.
The sombre tone of the black vestments can be understood as a reminder of our sin (personal and original) and the reality of death. It emphasizes the reality of purgatory and the need for prayers which we should offer for the dead. Also, we, the living, are thus reminded of the need to care for the state of of our own souls.
Another symbolic tradition is the use of a catafalque or cenotaph representing a coffin for the deceased members of the world. It is covered with a black cloth or pall, on which there is a cross either of white or some other color. The catafalque is usually placed immediately outside the sanctuary, and is the center of the ceremonies of that part of the Mass known as the absolution, receiving the same attention as the corpse would if present. So, candles are burned all around the catafalque during the function, and it is blessed with holy water and then incensed.
Here is a beautiful video of a Solemn High Requiem Mass on All Soul's Day at St. John Cantius in Chicago. While the Mass is being prayed, Mozart's Requiem Mass is being preformed by choir and orchestra in the balconies.