One Direction closed out their big 1D Day recently with a special guest -- Michael Buble!

The Canadian crooner joined the band via satellite from a Las Vegas concert for a duet of 'White Christmas.' The self-proclaimed "Christmas prostitute" is infamous for his beautiful holiday tunes, and he was complemented as such by the British boys.

The performance was a stately closer to the band's seven-hour streaming celebration, which also featured Jerry Springer, wrestler Kurt Angle and more.

However, the ensemble grew even further when the entire crew of the show joined in for a sing-along -- and it happened only about thirty seconds into the performance! It didn't sound quite as lovely with all those other non-professional voices in the mix, but it was still pretty cute to watch. A very merry time was had by all!