He gave up his last leadership post this year when he stepped down as head of the ruling Communist Party and has retreated further and further from public view.
His gradual slipping away appears to be a product of choice borne of necessity, but also of a transition plan to wean Cuba from its once near-total dependence on the charismatic Comandante's leadership.
He is rarely seen or heard from and has stood largely on the sidelines as his younger brother and replacement, President Raul Castro, struggles to reform Cuba's Soviet-style economy.
"His role has diminished significantly. He has stepped away more so than at any point in the last five years," said Jonathan Benjamin-Alvarado, a Cuba expert at the University of Nebraska in Omaha.
"It may be a case of Raul Castro solidifying his governing style and Fidel willingly receding," he told Reuters.
Cuba was to celebrate the birthday on Saturday with a nationally televised "serenade" by a lineup of musicians. Organizers said this week they did not know if Fidel Castro would attend personally.
He came to power on New Year's Day 1959 when his guerrilla forces swept down from the eastern Sierra Maestra mountains to topple U.S.-backed dictator Fulgencio Batista.
As Cuba's president, he outlasted nine U.S. presidents and five decades of U.S. hostility, but in July 2006 he underwent emergency intestinal surgery and suffered complications from which he never fully recovered .
He handed power provisionally to Raul Castro, then did so officially when he resigned in February 2008 and his younger sibling was elected president by the National Assembly.
The fading of Fidel's political presence has mirrored a physical decline that was most notable at a Communist Party congress in April when he made just one appearance and had to be helped to his chair on the stage.
The man once famous for his hours-long speeches sat wordlessly as his brother did all the talking in a silent passing of the torch.
TENDING TO SICK CHAVEZ
A year ago, when he reappeared in public after four years of semi-seclusion, he was vigorous enough to launch a campaign warning the world that U.S. moves against Iran's nuclear capability could lead to a nuclear holocaust.
Now, like many elderly, he is tending to the health of a sick friend, in this case close leftist ally Venezuelan President Hugo Chavez who is being treated in Cuba for an undisclosed type of cancer.
Fidel Castro has not been seen since the party congress except in videos chatting with Chavez, in which his physical frailty was clear, but he still seemed mentally alert.
Chavez' illness inspired the only opinion column, or "reflection," by Fidel Castro this summer. Until three months ago, the old revolutionary had written regular opinion pieces on world affairs assiduously published by Cuban state media.
In his July 3 piece on Chavez, in which he predicted the Venezuelan leader's full recovery, he said he had "momentarily" been writing less because he was "attending to other matters that are now top priority." He gave no more explanation.
After his five years out of power, life without Fidel Castro is not as unimaginable for Cubans as it once was.
He is loved by some and hated by others, but increasingly Raul Castro, 80, has supplanted him as the man considered critical to the future of the Revolution. His importance is magnified by the lack of younger potential leaders under him.
"People used to worry about what would happen if Fidel died, but now it's Raul. Raul replaced Fidel, but who will replace Raul?" said mechanic Rafa Marrero.
Nevertheless, Benjamin-Alvarado said Fidel Castro's death will be a "historical moment" for Cuba and perhaps the catalyst for more change.
Whether they like Fidel Castro or not, Cubans "are ready to move on. So that will be the point where there will be pressure on Raul to extend the changes he has initiated," he said.