Compiled by Gerhard Peters
"Whenever the President transmits to the President pro tempore of the Senate and the Speaker of the House of Representatives his written declaration that he is unable to discharge the powers and duties of his office, and until he transmits to them a written declaration to the contrary, such powers and duties shall be discharged by the Vice President as Acting President."
-- Constitution of the United States, Amendment 25, Section 3. [Ratified February 10, 1967]
The following table lists all instances when Presidents have invoked the disability clause of the 25th Amendment. Prior to this Amendment, however, on some occasions Presidents have announced planned hospitalizations in advance. They have specified "procedures in the event of Presidential inability."
These include from the Johnson Administration "Statement of Procedures for Use in the Event of Presidential Inability," October 5, 1965. These procedures were used by both Eisenhower and Kennedy. Essentially the Vice President would serve as Acting President during the period of inability. Johson submitted a Special Message to Congress on Presidential Disability in January 1965, urging adoption of resolutions for a Constitutional Amendment.
All APP documents with references to "Presidential disability" or "Presidential inability" can be retrieved with this search link.