One of the features of the Extraordinary Form is the ability to pray additional sets of Collects (opening prayers), Secrets (prayers over the oblations), and Postcommunions (closing prayers) in addition to the main prayers for a particular Mass. These additional sets of prayers are called Commemorations. By means of these Commemorations, the Church is able to keep multiple liturgical observances in the celebration of one Mass.
There are two types of Commemorations: Privileged and Ordinary. Privileged Commemorations are made in all Masses (and also at Lauds and Vespers), while Ordinary Commemorations are made only at Low Mass (and at Lauds).
The prayers for Advent are Privileged Commemorations, so even if the Church is observing a Saint’s Feast Day, this season is always observed in the daily liturgy.