According to Luke's gospel, the angelic annunciation of the birth of the Savior of the world came not to important dignitaries or kings, but to shepherds tending their flocks in the middle of the night. While the recipients of the message were certainly important to God's plan, equally so were the sheep they watched.
That shepherds would be out with their flock in the middle of winter should not be a surprise, since Israel's winters are Mediterranean (usually cool and short). Rainfall totals increase from December through February, but Jewish history suggests that the flocks remained even through the rain. In fact, these sheep stayed in the fields thirty days before Passover (February) when the clouds unleash their worst torrents.
We can imagine, then, the somber conditions into which angelic light blazed to life with a message not heard since the days of Isaiah the prophet. Although we know very little about these shepherds, they likely did not observe religious practices, since their isolation in the fields and the necessity of their constant attention made this impossible. But their lack of religious obligations doesn't mean their service was strictly secular.
Somewhere deep in Jewish tradition (revealed in writings called the Mishnah), a belief had arisen that the Messiah would be revealed from the Migdal Eder ("the tower of the flock"). This tower stood close to Bethlehem on the road to Jerusalem, and the sheep that pastured there were not the type used for ordinary purposes. The shepherds working there, in fact, took care of the temple-flocks, the sheep meant for sacrifice.
We can trust that God had a specific purpose for this shepherd audience, and the work they performed suggests the reason. These men who watched the sheep meant for the slaughter received a divine message about the ultimate Lamb who would take away the sins of the world through His death and resurrection.
Adapted from The Life and Times of Jesus the Messiah by Alfred Edersheim (Book II, Chapter VI).