Don't forget to pray for the souls of the dead today. Here's a refresher on the doctrine of purgatory from Catholic Answers.
Far from being a medieval invention, praying for the dead was a Jewish practice that pre-dated Christ by at least a couple of centuries. The early Christian Fathers believed in purgatory, though they did not treat it in the detailed, systematic way that the medieval theologians did. C.S. Lewis believed in purgatory -- "The Screwtape Letters" contains a reference to it in the final chapter.