In the Church calendar, 2 November is celebrated as the feast of All Souls, but the entirety of November is traditionally dedicated to the Holy Souls; that is, it is a month in which we specially remember and pray for all the souls in Purgatory.
As humans we are a body-soul hybrid, and at death our bodies die but our souls live on. While there is no way of knowing for definite whether our deceased loved ones are in Heaven, Hell or Purgatory, we pray for them anyway. It is wiser to assume they are in Purgatory and therefore we will pray for them, rather than assuming that they are already in Heaven, and not to pray for them.
The “holy souls in Purgatory” are the souls of people who have died in God’s grace but still need to be purified before entering heaven.
Just like we help our friends and family here on earth, we can help the souls in purgatory by offering our prayers, sacrifices, and Masses for them. The Church calls this one of the spiritual works of mercy — to pray for both the living and the dead.
In the month of November families can visit a cemetery and pray for the dead, create a list of loved ones who have died, display photos of loved one, light candles for them at Church, dedicate the rosary for them, have masses said for them and pray the traditional Requiem Prayer for the dead:
“Eternal rest grant unto them, O Lord, and let perpetual light shine upon them. May the souls of all the faithful departed, through the mercy of God, rest in peace. Amen.”