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Baby's first Christmas!

Sandra Bullock and little boy Louis Bardo -- he turns one next month -- got into the holiday spirit over the weekend.

Single mom Bullock, 46, helped her adopted son celebrate his very first Christmas season, arriving at a yuletide bash of a friend in Tujunga, Calif. Saturday.

"Sandra was in a great mood," a witness tells UsMagazine.com. During the low-key bash, little Louis took a pony ride, donned a Santa hat, and even sat on Santa Claus' lap.

"Everyone was commenting on what a sweet and mellow baby he is," the observer marvels. "It looked like they had a great time!"

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