Why might Nathaniel Hawthorne, in The Scarlet Letter, use the phrase "But, one idle rainy day . . . "?
The Scarlet Letter, by Nathaniel Hawthorne, is generally perceived as a weighty, even depressing, story about sin and the devil in a dark Puritanical setting, and most readers do not bother to read the introductory story called "The Custom House." The quote in your question comes from this introduction, and it reads in full:
But, one idle and rainy day, it was my fortune to make a discovery of some little interest.
Nathaniel Hawthorne, in this lengthy introduction, makes this statement as if it carries no importance or significance. It is a simple declaration of something that happened once on an "idle, rainy day," yet the description which follows suggests is that his discovery was in fact quite momentous for him.
[T]he object that most drew my attention...was a certain affair of fine red cloth, much worn and faded.... This rag of scarlet cloth,--for time, and wear, and a sacrilegious moth, had reduced it to little other than a rag,--on careful examination, assumed the shape of a letter. It was the capital letter A.... It had been intended, there could be no doubt, as an ornamental article of dress; but how it was to be worn, or what rank, honor, and dignity, in by-past times, were signified by it, was a riddle which I saw little hope of solving. And yet it strangely interested me. My eyes fastened themselves upon the old scarlet letter, and would not be turned aside.... I happened to place it on my breast. It seemed to me,--the reader may smile, but must not doubt my word,--it seemed to me, then, that I experienced a sensation not altogether physical, yet almost so, as of burning heat; and as if the letter were not of red cloth, but red-hot iron. I shuddered, and involuntarily let it fall upon the floor.
Hawthorne also discovered an old scroll which recounted some particulars about a woman named Hester Prynne and a certain scarlet letter. From these two things, Hawthorne writes The Scarlet Letter. His rather idle discovery turned into something significant.
In addition to being an ironic understatement, the quote is also similar to the way a fairy tale might begin. While The Scarlet Letter is certainly not a fairy tale, it is a moral tale created from a scrap of fabric, a few lines on a piece of paper, and Hawthorne's imagination. He tells us plainly that he has made up this tale:
I have allowed myself, as to such points, nearly or altogether as much license as if the facts had been entirely of my own invention.
In both his novels and short stories, Hawthorne's syntax and sentence structure are complex; we should expect no less complexity from any "simple" statement he makes about himself and his inspiration for The Scarlet Letter.