Katharine McPhee, David Foster Perform Christmas Songs Together at Holiday Party

David Foster and girlfriend Katharine McPhee seem to know that the couple that plays together one-horse open sleighs together ... hey, it's that time of the year.

Dave and Kat were just two of many celebs that attended singer-songwriter Carole Bayer Sager and her hubby Bob Daly's holiday party Saturday night ... but it appears they were also among the few that actually performed onstage.

Kim Kardashian caught the magic on camera, showing D&K doing a rendition of "White Christmas" with the help of Babyface -- and then pairing off to do Kelly Clarkson's "My Grown Up Christmas List." Dave handled piano -- and, naturally, Kat was on vocals.

Other musicians like Keith Urban and Joe Walsh sang Xmas ditties as well, and were even joined by the likes of Hugh Jackman at one point.