This was the first biggest hit by Glenn Miller in 1939. It's a ballad of 2 lovers who are spending a romantic night together.
With this song, Miller created a new sound with a high-leading clarinet leading over the saxophone section. That kind of sound made Glenn Miller's orchestra famous. If you listen the song you'll hear that there are actually 2 melodies (clarinet and saxophone), and together they're creating the whole song.
Suggestion credit: zabavnik - ljubljana, for above 2