You go back to 1914 and redirect the Archduke Franz Ferdinand's car on a different route, and without killing anyone, you may stop both world wars, and we never even hear of about anyone called Hitler.
The birth of the Renaissance in 15th Century Florence, home of the Medici's, De Vinci, Michelangelo, Raphael, Brunelleschi. I think I'd pick the week Brunelleschi finally finished the dome on the Santa Maria del Fiore, at this point De Vinci took over the design of the lantern at the top, so...