Pope Emeritus Benedict XVI on Tuesday celebrates his 86th birthday as the first former pope in over 700 years at his temporary home of Castel Gandolfo, the papal summer residence near Rome.
His successor Pope Francis called to wish him a happy birthday and celebrated mass in his honour.
Francis had a "cordial" telephone conversation with his predecessor, the Vatican said in a statement.
Benedict was joined by his older brother Georg, who is 88 and nearly blind, as well as the four Italian religious laywomen who have worked for him as housekeepers for years.
Benedict in February became the first pontiff to resign in more than 700 years in a move that stunned the world. A conclave of cardinals elected Francis in March.
He has lived as a recluse in Castel Gandolfo some 25 kilometres (16 miles) south of Rome since February 28, never leaving the palace.
Benedict's secretary Georg Gaenswein, who is also head of the pontifical household for Pope Francis, is also expected to be present for the birthday, which will be an entirely private event.
The former pope spends his days praying, doing academic research and playing the piano in the palace on a hillside above Lake Albano, the Vatican said.
The Holy See last week denied that Benedict had a serious illness, following rumours in the Spanish press.
The most recent images of the former pope showed him walking slowly with the help of a cane.
The Vatican has said Benedict will eventually move to a former convent on a picturesque hilltop inside the Vatican walls to live out his days.