Just as we have All Saints Day (Nov 1) in the Catholic Church where we celebrate those who have gone before us who we consider saints, we take an additional day (Nov. 2) to pray for all of those who have gone before us who may not yet be saints in heaven with God.
While All Souls Day is not a Holy Day of Obligation, it is an opportunity that many take to pray for the souls of those who have died over the past year. While we have hope that all those we love are with God in heaven, it is not a certainty. So we pray for those in purgatory especially on this day, that they might soon be joined with God forever.