The entire month of November is devoted to the Poor Souls in Purgatory.
The holy souls suffer in purgatory to expiate either their venial sins, or those mortal sins, which, though absolved, have not been completely atoned for. 
The Catechism Explained: An Exhaustive Explanation of the Catholic Religion by Rev. Francis Spirago, edited by Rev. Richard F. Clarke, SJ (Benziger Bros. NY, 1899)