“Give every day the chance to become the most beautiful day of your life.”

Quoted in A Living Architecture : Frank Lloyd Wright and Taliesin Architects (2000) by John Rattenbury
Context: Human beings can be beautiful. If they are not beautiful it is entirely their own fault. It is what they do to themselves that makes them ugly. The longer I live the more beautiful life becomes. If you foolishly ignore beauty, you will soon find yourself without it. Your life will be impoverished. But if you invest in beauty, it will remain with you all the days of your life.

A.A. Attanasio. Centuries. 1997. p.310 ISBN 978-1-60450-283-1

The day I stood up alone
Context: There are two most powerful days in your life: the day you're born, and the day you discover why. That day standing up in that stadium shouting at the President, I discovered why I was truly born, that I would no longer be silent in the face of injustice.

This appears on the opening placard of the film The Equalizer, attributing it to Twain, but there is no evidence that Twain wrote it. A precursor is found in Taylor Hartman's self-help book The Character Code (first published 1991), where it is not attributed to Twain: "The three most significant days in your life are: 1. The day you were born. 2. The day you find out why you were born. 3. The day you discover how to contribute the gift you were born to give" ( Google Books link https://books.google.com/books?id=gIKCxWxNmeMC&pg=PA147&dq=%22day+you+find+out+why%22&hl=en&sa=X&ved=0ahUKEwijrJzc84vLAhUJzGMKHajvADEQ6AEIIjAB#v=onepage&q=%22day%20you%20find%20out%20why%22&f=false)
Disputed

Source: Maria Chapdelaine (1913), Ch. 6, p. 86

Source: The Art of Thinking (1928), p. 250