The mood is sleepy, drowsy, dreamlike
the sleepiness of the town and its contagious nature lures us right into Irving's tale. It's like when stories begin, "It was a dark and stormy night," and you know exactly what kind of story it's going to be. The setting becomes a persistent feature of the story, and we know it's always there, giving off its sleepy vapors, making people dream dreams.
I hope this helps!