Well, Christmas is actually an ancient holiday that originated in Rome around the year 336 AD. However, the modern Christmas traditions that we know and love today, like Santa Claus, Christmas trees, and gift-giving, were popularized in the 19th century by German immigrants to the United States. So, while there is no one definitive answer to this question, we can say that the modern celebration of Christmas as we know it was largely established in the United States.