1945 – U.S. President Franklin D. Roosevelt dies while in office; Vice President Harry Truman, becomes President upon Roosevelt's death.
https://en.wikipedia.org/wiki/April_12Not sure might be something else, but anyways I don't see any purpose having a holiday on April 12th.
edit: Could possibly be this:1864 – American Civil War: The Battle of Fort Pillow: Confederate forces kill most of the African American soldiers that surrendered at Fort Pillow, Tennessee.
Anyways Awful reasons to have a holiday on that date.