Library Question #7

On a quick sweep through a second-hand bookstore in Concord, Massachusetts, you come upon a hitherto unknown letter by Henry David Thoreau wedged behind a tottering stack of The Youth's Companion from the 1890s.

Internal evidence in the letter proves that it was written between 1849 and 1856. It is dated, at the head of the page, simply "Friday, May 5."

What was the year?

Describe the process by which you arrived at your conclusion. Cite your sources and -- as I've said before -- remember to assess the credibility of the sources -- their accuracy, thoroughness, and scholarly integrity. 

Good luck.