First, from last year, Reading the O Antiphons Forward
And then from this year, The Other Major Antiphons for the End of Advent, from which this quote is taken,
The best known feature of the Office in Advent is of course the O Antiphons, which will be upon us on Sunday, said with the Magnificat from December 17-23. Their prominence has perhaps overshadowed some of the other riches of the season, which has an unusually large number of proper texts. In addition to the daily antiphons of the Benedictus and Magnificat, the psalms at Sunday Matins also have their own antiphons, which is not true of either Lent or Passiontide, and each individual Sunday has another set of five antiphons for the psalms of Lauds and Vespers, also used at the minor Hours of the day.