The Month of November is dedicated to the Holy Souls in Purgatory. From Catholic Culture:
Since the 16th century Catholic piety has assigned entire months to special devotions. As a reminder of our duty to pray for the suffering faithful in Purgatory, the Church has dedicated the month of November to the Holy Souls. The Holy Souls are those who have died in the state of grace but who are not yet free from all punishment due to their unforgiven venial sins and all other sins already forgiven for which satisfaction is still to be made. They are certain of entering Heaven, but first they must suffer in Purgatory. The Holy Souls cannot help themselves because for them the night has come, when no man can work (John 9:4). It is our great privilege of brotherhood that we can shorten their time of separation from God by our prayers, good works, and, especially, the Holy Sacrifice of the Mass.
Not only is praying for the dead a spiritual work of mercy, but the Church also offers a plenary indulgence for those who do this during the first week of November. Click here to learn more.