They were the lowest of the lowest - night shift shepherds, but to them came the news that Christ was born - angels singing "Glory to God in the highest, and on earth, peace, goodwill toward mankind."
After they saw Christ, the shepherds ran through the streets of Bethlehem telling everyone about this child born in a stable. This is the joy that comes when your faith has become reality - an overwhelming sense of awe and wonder, that God has fulfilled His promises.
The Jews had waited for hundreds of years for their Messiah to come, and after a hundred plus years of silence from God, the Messiah arrived.
Joy is not happiness for one can have joy and be crying in the deepest sorrows. Joy is that uncompromising peace and knowledge that God fulfills His promises not because we think He will, but because we know He will. We have seen Him fulfill His promises. Joy is the awe and wonder that comes from seeing a miracle.
As we celebrate the season of Advent, we remember that joy the shepherds had when all that they had waited for had become reality: Immanuel - Christ with us.