According to The Writer's Almanac, today is Ralph Waldo Emerson's 209th birthday. I felt tortured by having to read him in college, but I've always enjoyed reading about 'The Sage of Concord' and his group of friends.
They also quoted these lines, which I kept tucked into my desk drawer at my old job:
"Finish every day and be done with it. You have done what you could; some blunders and absurdities no doubt crept in; forget them as soon as you can. Tomorrow is a new day; you shall begin it serenely and with too high a spirit to be encumbered with your old nonsense."