"When Christmas Comes" is a song by American singer and songwriter Mariah Carey from her second Christmas album/thirteenth studio album, Merry Christmas II You (2010). Carey wrote and produced the song in collaboration with James Poyser. A soul song with an influence of R&B, the lyrics are about giving the gift of love. In November 2011, Carey re-recorded the song as duet with John Legend, which was later released as a single. Both versions of the track were a hit in South Korea, with the duet debuting at number one with sales in excess of 80,000 copies. The song's accompanying music video features Carey and Legend at a hosting a Christmas house party. It has been performed by Carey at her ABC 2010 Christmas special and during her 2014 Beacon Theatre residency All I Want for Christmas Is You, A Night of Joy and Festivity in December 2014.

"When Christmas Comes" was written and produced by Mariah Carey and James Poyser for the former's second Christmas album/thirteenth studio album, Merry Christmas II You (2010). Her vocals were recorded by Brian Garten at Guardian Angel Studios in Bel Air, Westlake Recording Studios and Hensons Studios, both located in Los Angeles, and Germano Studios in New York City. Carey performed her own background vocals along with Melonie Daniels, Sharlotte Gibson and Nicki Richards. The music for the song was recorded by Garten and Kevin Guarnieri; Alex Evans played the drums, while John "Jubu" Smith played the guitar. On November 7, 2011, Carey revealed that she would be releasing "When Christmas Comes" as a duet with John Legend. The duet was released in the United States on November 21 and the United Kingdom on November 28.

It as an R&B-influenced soul song with a "simple groove" that lasts for a duration of four minutes and forty-six seconds. The single release version with Legend lasts for additional second. The instrumental consists of horns, including the trumpet performed by Rick Baptist and Greg Adams, and the trombone by Nick Lane. Other brass instruments include Dan Higgins on the sax and flute. In addition to being a producer, Poyser performs the keyboard. Lyrically, the song is about "giving the gift of love", as Carey and Legend harmonize the lyrics "And me and you gon' have ourselves a holiday/ And we don't need nobody else to celebrate/ And we're gon' kiss our worries and our cares away/ I can't wait/ Because this Christmas time, get together / It's gonna be so nice, better than ever/ And baby you’re the one, special treasure." Prior to the final chorus, the pair freelance the a portion of the lyrics from "Jingle Bells". As noted by Scott Shelter of PopCrush, "listeners are treated to notes from the upper reaches of Mariah’s five-octave range, which she rarely shows off anymore." In an interview for Vevo, Carey stated that she felt that Legend was the "perfect choice" to re-create the song as a duet with, and that his "soulful" creative decisions "took it up several notches".

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