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.