how did saturnalia become christmas

"Saturnalia was a Roman pagan festival that was celebrated during the winter solstice. It was a time of great feasting and merriment, and many of the customs associated with Christmas today, such as the exchange of gifts and the decoration of the home, originated from Saturnalia. However, as Christianity spread throughout the Roman Empire, the celebration of Christmas gradually took on a more religious significance, and many of the pagan elements of Saturnalia were incorporated into the holiday. Today, Christmas is celebrated as a primarily Christian holiday, but it still retains many of the cultural traditions that were originally associated with Saturnalia."