In the gospel of Matthew and Luke is the story of Mary and her husband, Joseph, who were the parents of Jesus. Jesus was sent to Mary by God, and asked to bear him as her own child. Mary and Joseph were in the town of Bethlehem when it was time to give birth, but they could find no room in any of the inns in the town. Therefore, they did the best they could, and found a stable in which to find protection from the elements. There, Mary gave birth to Jesus, wrapped him in clothes, and placed him in a manger.
The birth of Jesus fulfilled the prophecies of the Old Testament and set in motion all the things that would come in the three decades afterward, including all the teachings of Jesus, leading up to his death and promise to rise again.
Christmas is celebrated on December 25th in most corners of the world, and is designated as the celebration of the birth of Jesus. Some places celebrate Christmas on January 7th, in keeping with different calendars. Some even celebrate Christmas in July, in the belief that the date of conception happened nine months prior. Some believe that the date of Christmas was a random date chosen to mark the Winter Solstice. Regardless of when it is celebrated, the reason for the celebration is clear.
The tradition of a Christmas tree in the house actually began in Roman times, when trees were brought into homes for protection from the elements during the harshest weather. Pagan traditions of decorating trees soon became adapted to Christmas rituals, and so did the hanging of the lights. Centuries ago, European countries began celebrating Christmas by decorating their homes with anything green — evergreen boughs, pine needles, and ivy were popular options. It was believed that green denoted life, even in the cold and dreary winters, and was seen as a fitting way to honor the season.
Regardless of the date Christmas is celebrated or the way in which it is observed, there is no doubt that the birth of Jesus is the reason for the season.