Advent is the time of year we wait in eager anticipation of the ChristMass Season.
ChristMass begins with Midnight Mass, of course at midnight during the first minutes of December 25th. This is truly when we should BEGIN the celebration of ChristMass!
This season lasts for twelve days - till the Epiphany, on January 6th. It is these twelve days that we should be celebrating the birth of our Lord.
Why do we celebrate in late December and early January though?
Early Christians sought to calculate the date of Christ's birth based on the idea that Old Testament prophets died either on an anniversary of their birth or of their conception. They reasoned that Jesus died on an anniversary of his conception, so the date of his birth was nine months after the date of Good Friday, either December 25 or January 6. Thus, rather than the date of Christmas being appropriated from pagans by Christians, the opposite is held to have occurred. [See Duchesne (1902) and Talley (1986).]So, based on these calculations - it seems understandable why the ChristMass Season lasts from December 25th through January 6th. This also coincides with the time frame many Orthodox celebrate ChristMass - being January 7th. Their reason for doing so on that date is because they did not recognize "the reforms of the Gregorian calendar nor the Revised Julian calendar" (ibid).
Advent is nearly over - and we eagerly await the wee hours of the morning of December 25th, and the First Mass of the day - the Christ Mass. (Noting also, that's why it has the name "Christmas" - in remembrance of the "Christ Mass").
A blessed Advent to all, and I hope your ChristMass is wonderful!