ROME (AP) — Pope Francis was resting Thursday after a peaceful night during his third week of hospital treatment for double pneumonia, the Vatican said.

The pope has been sleeping with a non-invasive mechanical mask to guarantee that his lungs expand properly overnight and help his recovery. He has been transitioning to receiving oxygen with a nasal tube during the day.

The 88-year-old pope, who has chronic lung disease and had part of one lung removed as a young man, has been stable for two days after suffering a pair of respiratory crises Monday. Doctors underlined that his prognosis remained guarded due to the complex picture.

The pope on Wednesday marked the start of Lent by receiving ashes on his forehead and by calling the parish priest in Gaza, the Vatican said. He also added physical therapy to his hospital routine of respiratory therapy.

The Catholic Church opened the solemn Lenten season leading to Easter without the pope's participation. A cardinal took his place leading a short penitential procession between two churches on the Aventine Hill, and opened an Ash Wednesday homily prepared for the pontiff with words of solidarity and thanks for Francis.

“We feel deeply united with him in this moment,″ Cardinal Angelo De Donatis said. ”And we thank him for the offering of his prayer and his suffering for the good of the entire church in all the world."

On Ash Wednesday, observant Catholics receive a sign of the cross in ashes on their foreheads, a gesture that underscores human mortality. It is an obligatory day of fasting and abstinence that signals the start of Christianity’s most penitent season, leading to Easter on April 20.

“The condition of fragility reminds us of the tragedy of death,″ De Donatis said in his homily. ”In many ways, we try to banish death from our societies, so dependent on appearances, and even remove it from our language. Death, however, imposes itself as a reality with which we have to reckon, a sign of the precariousness and brevity of our lives.”

The pope was supposed to attend a spiritual retreat this weekend with the rest of the Holy See hierarchy. On Tuesday, the Vatican said the retreat would go ahead without Francis but in “spiritual communion” with him. The theme, selected before Francis got sick, was “Hope in eternal life.”