9 April

On 9 April 1865, the Fifth Doctor prevented U.S. President Abraham Lincoln from being assassinated by Aaron Eddowes, in Richmond, Virginia, the capital of the defeated Confederate States of America. (PROSE: Blood and Hope)

Behind the scenes
According to Clive Finch in TV: Rose, the Ninth Doctor took a picture with the Daniels family, and convinced them not to board the Titanic the following day, sometime in April 1912. Although it is not stated in the episode, the Titanic set sail on 10 April, making those events take place on the ninth.