For your easy reference, the following is a list of weeks and years with their corresponding date:
20 Dec 2015 – 4th Sunday of Advent Year C
21 Dec 2015 – 21 December – Season of Advent
22 Dec 2015 – 22 December – Season of Advent
23 Dec 2015 – 23 December – Season of Advent
24 Dec 2015 – Christmas Eve – Midnight Mass
25 Dec 2015 – Christmas Day – Mass During the Day