The Latin word for rejoice even the liturgical colors change for this one day. The theme is reflected in the the readings. The first reading tells us to shout for joy to sing joyfully. The psalm tells us to cry out with joy and gladness. In the reading from the Philippians, we are told to rejoice in the Lord always. We know that Christmas is coming, the day that we celebrate the birth of Christ.
But for the people of Israel it wasn't so. John the Baptist had come along and they were filled with questions. What should we do? Are you the Christ? These are common questions. Since the beginning of time people have looked for leadership, they have looked for a savior. In some cases the chosen leader has lifted up a nation. But we have many examples where a charismatic leader has lead nations to destruction, but we still keep looking.