"Rejoice" is the first word of the Entrance Antiphon for the third Sunday of Advent.  In the old liturgy this Sunday was called by the Latin form of that word: "Gaudete Sunday."  In those days the nature of the season of Advent was thought of as being primarily penitential.  So Gaudete was somewhat similar to Laetare Sunday in Advent, the "past the half-way point".  It signaled anticipated joy that we're almost there.  Or, as G.K. Chesterton had expressed it, the human spirit cannot be held down for too long; sooner or later it pops up again with joy. 

We no longer place as much emphasis on penance and mortification during Advent as before.  We still heed the message of John the Baptist to "Repent, make straight the ways of the Lord." and we try to cleanse ourselves for the coming celebration of God's breaking into our human world in a definitive manner.  But at the same time we place emphasis on the "anticipated joy" of Christmas.  When I was a small boy and my father would promise me during a hot afternoon out in the hayfield "If we can finish getting this hay in the barn by Saturday, we'll go to the lake on Sunday."  Since my father was man who kept his promises, my brothers and I put in extra efforts to be sure that we would be finished by Saturday.  But I could already feel the cool lake water refreshing my body, even as the rivers of sweat ran down my back on that hot, dusty hay field.  There was a popular song in those days "Anticipation... is driving me crazy" which later became background to a TV add showing an upside down bottle of Heinz ketchup as "the slowest ketchup in the West" slowly prepared to descend on an awaiting hamburger.