3rd Sunday of Advent: What is Joy?

The 3rd Sunday of Advent is known in the Church as "Gaudete" Sunday, which translates, "Rejoice!" Our readings today definitely have this theme of joy -- not as a suggestion or an invitation but rather as a command: "Rejoice!" Paul says, "Rejoice in the Lord always; again, I say, rejoice!" Since Paul writes these words from prison, we should understand that he is telling us to rejoice at all times, whether we are having a good or bad day, whether we are flourishing or struggling. How do we distinguish true joy from happy feelings? And can we live with authentic, Christian joy, even when experiencing the hardships of life?