An all-instrumental album by Willie Nelson and his road band? Sounds strange — but it’s just traditionalism. When Nelson was just another Nashville songwriter in the early 1960s, instrumental country albums were not uncommon, done by studio hotshots like Chet Atkins or sometimes crack backup bands like Buck Owens’ Buckaroos. The hippest collections offered country-toned versions of show tunes and jazz standards, just like Night and Day does. Given that Nelson and his seasoned gang are the masters of the offhand seduction, the workouts here glow with the friendly feel of a deeply coherent band. On “Vous et Moi,” the first track, the instruments slide in behind the leader’s guitar like a stream finding its way downhill. With Mickey Raphael taking particularly voicelike harmonica solos, a bittersweet Spanish tinge is everywhere, from the title tune to the Nelson original “Bandera.” Although Night and Day arrives without the usual Saint Willie fanfare, if there’s any justice, it won’t become another of his lost low-key triumphs, like the venerable Face of a Fighter or the recent Just One Love.