The season of Advent marks the time of preparation and hope for the coming of the Christ in Western Christianity. 

The traditional Advent period begins on the fourth Sunday before Christmas Day, and it lasts through Christmas Eve (December 24), unless Christmas Eve falls on a Sunday, in which case, Christmas eve becomes the fourth Sunday of Advent. But there is much more to know about Advent, and below we've gathered five important facts about the beginning of the Christian Christmas season. 

1. Advent Start Dates Differ Each Year

The start of Advent changes from year to year. Though many assume that Dec. 1 is the start of the Advent season, this isn't always the case since it begins on the Sunday that is closest to Nov. 30, according to Patch. This also affects the length of the season. 

This year, the four Sundays of Advent are Dec. 1 (First Sunday of Advent), Dec. 8 (Second Sunday of Advent), Dec. 15 (Third Sunday of Advent), and Dec. 22 (Fourth Sunday of Advent).

2. There Are Christian Hymns Specific To The Season

The most famous hymn of Advent is "O Come Emmanuel," according to the Huffington Post. The song's lyrics are about the hope that the season brings and are based on the words of the Prophet Isaiah. The word advent -- which comes from the Latin adventus -- means “the coming.”

3. Advent Wasn't Always An Annual Tradition 

Although it is now a traditional part of the Roman Catholic, Anglican, Lutheran, Moravian, Presbyterian and Methodist traditions, Advent didn't become routine until the fourth century A.D., when the Roman Emperor Constantine designated Dec. 25 as the official birthday of Christ. 

4. There Is A Color Code For The Season

While Advent is usually a time of prayer and repentance, it's also one for rejoicing and celebrating, and purple is the main color associated with it. The first two weeks are solemn, with the colors purple and blue used during those two weeks. The third Sunday of Advent uses pink or rose as its main colors; this represents a change in mood to cheer in preparation for the birth of Christ. 

In many churches, circular wreaths that hold five candles are also used as part of the Advent tradition. One candle is lit on each Sunday of Advent. The last candle -- known as the Christ Candle --  is lit on Christmas Eve.  

5. There Are Also Themes For Advent

The themes of Advent are hope, peace, joy and love.