From “Missale Sacri Ordinis Praedicatorum"; 1933
The ferial days in Advent have the rank of III Class, until December 17. During these days, when a III Class feast occurs in the Proper of the Saints, a commemoration is made of the ferial day at Lauds and Vespers. From December 17 through December 23, the ferial days have the rank of II Class. During this period, when a III Class feast occurs, the ferial office is prayed and a commemoration of the III Class feast is made at Lauds and Vespers.