The Church has always advocated that we prepare for our feasting first by fasting, and so the third week of Advent (and week of the feast of St Lucy) traditionally includes the Advent Ember Days.
The days are Class II, with a rather more elaborate mass.
For Advent the focus is particularly on Our Lady. Wednesday's Mass is about the Annunciation. It starts with the beautiful Introit Rorate Caeli (Drop down dew ye heavens) and includes the famous prophesy from Isaiah (Behold a virgin shall conceive) as well as the Gospel from St Luke with the angel Gabriel appearing to Mary. Friday's Gospel is about the Visitation. Saturday's ancient and complex Mass Mass is a more general message about the preparation for the coming of Our Lord, with the Gospel on St John the Baptist.
So do try and get to a Mass on these days if there is one that celebrates the Ember Days in your area (they are optional in the Novus Ordo calendar).
And remember that they are also traditionally days of fasting and abstinence.