The note was read aloud to the hundreds of people in the Karl Marx Theater, The Miami Herald reported. Castro said he was "in no condition, according to the doctors, to face such a colossal gathering."
Castro announced four months ago that he was handing power temporarily to his brother, Raul. Although the government has said he had intestinal surgery, officials have been tight-lipped about the details of his illness.
The guest list for the gala included Colombian novelist Gabriel García Marquez, Bolivian President Evo Morales, Mexican actress Maria Rojo and former Ecuadorean President Rodrigo Borja, the Herald said.
In the 700-word note, Castro attacked President Bush, saying he had "created such a crisis" that U.S. voters would not let him complete his term. He also spoke of the need for energy conservation.
The climax of this week's festivities is a military parade planned for Saturday.