Thoreau went to the pond in July 1845, built a cabin on land Emerson owned, and stayed two years, two months and two days. Walden, published in 1854 after several rewritings, is not a diary of that stay but an argument built out of it, compressed into a single year that runs from summer through the ice to the spring thaw.
The argument is economic before it is spiritual, and the book opens with a table of expenses down to the half cent. His claim is that most people pay for their lives with their lives, working to support arrangements they never chose, and that the cost of a house, a diet and a wardrobe can be established experimentally by anyone willing to try. Having established it, he can ask what the remaining time is for.
The answer is a discipline without a church. Rising early, bathing in the pond, sitting in the doorway from sunrise to noon in what he calls a revery, watching the ice, measuring the depth of the water to settle a local legend that the pond was bottomless. He read the Bhagavad Gita there, and the passage in which he sets its philosophy beside the Ganges water and the Walden water, and imagines the servant of the Brahmin coming to his well, is one of the earliest serious American engagements with an Indian text. The seasons then supply the structure, and spring supplies the ending, on the ground that a life, like a year, is capable of beginning again.