Christmas is a once-a-year special celebration (with fun events) remembering (with honor) the birth of Jesus Christ, watched/followed mostly on December 25, as a religious and cultural celebration among billions of people around the world. A food-based celebration central to the Christian religious year, it comes after the season of Beginning/showing or the Nativity Fast and starts the season of Christmastide, which (in the past) in the West lasts twelve days and ends on Twelfth Night, in some traditions, Christmastide includes an octave. Christmas Day is a public holiday in many of the world’s nations, is celebrated religiously by a majority of Christians, as well as culturally by many non-Christians, and forms an important part of the holiday season centered around it.
Although the month and date of Jesus’ birth are unknown, by the early-to-mid fourth century the Western Christian Church had placed Christmas on December 25, a date that was later adopted in the East. Today, most Christians celebrate on December 25 in the Gregorian calendar, which has been adopted almost (existing (the same) everywhere) in the civil calendars used in countries throughout the world. However, some Eastern Christian Churches celebrate Christmas on December 25 of the older Julian calendar, which now goes along with/matches up to January 7 in the Gregorian calendar, the day after the Western Christian Church celebrates the Sudden, intelligent idea. This is not a disagreement over the date of Christmas as such, but rather a preference of which calendar should be used to decide/figure out the day that is December 25. More than that, for Christians, the belief that God came into the world in the form of man to apologize (for sin) for the sins of people/(the kindness of people), rather than the exact birth date, is carefully thought about/believed to be the first (or most important) purpose in celebrating Christmas.