Most Christians celebrate Christmas on December 25, but some do on January 6. The former follow the Gregorian calendar (named after Pope Gregory XIII) while the latter follow the Julian calendar (named after Julius Caesar). However, for everyday use, it is the Gregorian calendar that is now used globally. Interestingly, for events before 1582, the convention is to use the Julian calendar; and for events after 1582, it is to use the Gregorian calendar. So when we say Jesus was born in 5 BC or the crucifixion took place on April 3, 33 AD or Alexander the Great invaded India in 326 BC, we are using the Julian calendar. But when we say WW II ended in 1945, we are using the Gregorian calendar. The shift from Julian calendar to Gregorian calendar became necessary as the former no longer synchronized with the true solar year and therefore was getting out of sync with the seasons.
So despite differences in the date of celebration, Christians all over the world believe that Christmas is about God becoming human. Incarnation. But this is a one time event. If Christ was crucified on April 3, 33 AD, his resurrection was on April 5, 33 AD. The next time God returns, he won't do that as a human; he would do that as the glorious and majestic Lord.
But in his first coming, i.e incarnation, he was born in a manger and in a humble family. It was the shepherd that first received audience with the baby Jesus. Even before Jesus was born, Jesus' mother sang a song that looked forward to a future where the humble would be lifted up and the hungry filled (Lk. 1:52-53). And when Jesus reached around thirty, he began his 'work' by announcing that he was going to proclaim recovery of sight for the blind, release of the oppressed.... The point is that Jesus was deeply concerned for the poor and the less privileged.
Given the understanding that Jesus loved the poor and the weak so much, earliest Christians living in the Roman empire used to pick up discarded children from dung heaps who were left to die by their parents who would not raised physically disabled children. From time to time they would also pay to the slave master for the freedom of the slaves. These actions were driven by the conviction that God would want them to do these. Well, over the years, some people or rather many people lost this vision. So there were Christians who even owned slaves. It didn't register to them perhaps that slavery was an assault on human-ness; that human-ness which Jesus himself embraced. At the same time, politician like William Wilberforce, informed by the Bible, toiled for decades to ensure that slave-trading was abolished. And few centuries before this, monk like Telemachus went about preaching that gladiatorial game be banned, paying the price for his message with his own life which also led to the game being banned.
Today when Christians of certain stripes endorse white supremacy, Jesus would be appalled by such attitude. When certain Christians in India practice caste based discrimination, that is nothing short of heresy. The message of Christmas demolishes such discrimination.
Even today there are also many Christians who strive to remain faithful to teaching of Jesus in their own context. These people reach out to the poor with 'bags of rice'; these people reach out to the sick with medicines; these people reach out the underprivileged with education. These are ways of showing love.
Christmas is about God becoming human so that we human may have an "abundant life". And the application for the followers of Jesus is this: Christmas is about us incarnating into the lives of the vulnerable and the needy so that they may have an "abundant life".
