Advent always starts on the 1st Sunday closest to the Feast of St. Andrew. It can begin as early as Nov. 27th and as late as December 3rd.
It's also worth noting that on ferias (a "feria" is a day) the commemorations (i.e. the Collect, Secret, and Post Communion Prayers of the preceeding Sunday are added to the feast day commemorations.. Ember days (about more later) have a special Mass. In the case of Advent, the Ember days are the Wednesday, Friday and Saturday after the 3rd Week in Advent (Gaudete Sunday.)