The Feast of the Nativity commemorates the beginning of the end to darkness, that moment in time when the Saviour first appeared on earth to redeem us from our sins. But like the shepherds mentioned in the antiphon above, we must know who this Savior is in order to be transformed by worship of Him.
The Christmas season, or Christmastide , helps us towards this end. By openly contemplating the Light that was hinted at throughout the season of Advent, Christmastide recapitulates our Lord's gradual self-revelation to the world. Indeed, the order of Christmastide feasts is based on this principle.
The Nativity of our Lord marks the first moment that the Light shineth in the darkness. Naturally, this light will first be seen by those closest to it, which is why the Masses on Christmas and on the Sunday within the Octave of Christmas focus on the adoration of Christ's own people, the Jews.