My usual impressions of Tak Matsumoto and Koshi Inaba（松本孝弘・稲葉浩志） are of them just strutting their stuff on stage, especially with the latter in various stages of undress while the music is just bouncing off the speakers and the audience like a manic superball (I always think of "Bad Communication" and "Love Phantom"). But while the usual B'z touch is also infused into this Xmas song, it also comes across as one of those sway-worthy ballads that you and that significant other can enjoy in front of that Christmas tree.
"Itsuka no Merry Christmas" came out as the first track on B'z mini-album, "Friends" in December 1992. The album became a million-seller, selling a little over 1.3 million copies (according to Wikipedia), hitting the top spot on Oricon and becoming the 6th-ranked album of 1993. But the song itself managed to get its cellphone ringtone downloaded over half a million times, and the entire song was downloaded 100,000 times onto cellphones.