Purgatory is necessary so that souls can be cleansed and perfected before they enter into heaven. There is scriptural basis for this belief. The primary reference is in 2 Maccabees, 12:26 and 12:32. "Turning to supplication, they prayed that the sinful deed might be fully blotted out... Thus made atonement for the dead that they might be free from sin."
Consistent with these teachings and traditions, Catholics believe that through the prayers of the faithful on Earth, the dead are cleansed of their sins so they may enter into heaven.
All Souls Day is celebrated in much of the western world on November 2. All Souls Day is not a holy day of obligation. It should not be confused with All Saints' Day, which is a holy day of obligation.