One of the reasons Willie Nelson's recordings have had such a singular sound over the years is his loyalty to the core of musicians who've given his music a truly distinct personality. Of course, one of the reasons he's been so loyal to his pianist Bobbie Nelson is she happens to be his big sister, but whether it's blood or simply a common musical outlook, the gentle, intuitive interplay Willie and Bobbie share when they play together has long been one of the consistent pleasures of Willie's body of work. Music seems to come as naturally to Willie and Bobbie as drawing breath, and the two often jam on old favorites as their tour bus rolls to the next gig; Bobbie proposed the idea of cutting an album of tunes they love the way they play them on the bus, and 2014's December Day is the result. The notion of Willie and friends casually running through some old songs he likes isn't exactly novel -- Nelson spent much the '80s making records that sounded as if he knocked them out in a good afternoon -- but December Day does have a loose, Sunday afternoon feel that seems honestly familial, as if Willie and Bobbie just let the arrangements fall into place as tape rolled, and when the other players ease into the tunes -- including Mickey Raphael on harmonica, Billy English on drums, and Bee Spears on bass (who died shortly after cutting these sessions) -- it sounds less like seasoned pros making an album and more like a casual guitar pull with some old buddies. Along with old standards like "What'll I Do," "Mona Lisa," and "Always," Willie tosses in a few of his own tunes (including a new number, "Amnesia," about his difficulties with writing new songs, and another recent copyright, "Laws of Nature," that's a gentle but firm affirmation of his environmental principles), and "Sad Songs and Waltzes," "Who'll Buy My Memories," and "Is the Better Part Over" certainly gain a personal touch in these interpretations, though Willie has honestly sung them better in the past. December Day may be meant to offer an intimate look into the music Willie and Bobbie Nelson make for their own pleasure, and it does capture some marvelous moments, but given that Nelson had built a remarkable career out of mostly doing just what he feels like, this album, charming as it is, isn't as revealing as it might have been coming from other major country artists, though for Willie's die-hard fans, it's a must and it is a sweet reminder of how much Bobbie Nelson has brought to Willie's music over the years.