Former New York City Mayor Rudy Giuliani remained hospitalized Monday with pneumonia — with a longtime friend telling The Post, “Today is an important day.”

The 81-year-old’s shock weekend hospitalization came after he came down with pneumonia, said Giuliani’s business partner Tom von Essen, who served as the city’s fire commissioner during 9/11.

“Rudy had a tough weekend. He had pneumonia,” von Essen said.

Former NYC Mayor Rudy Giuliani was hospitalized with the pneumonia. Getty Images

“Today is an important day,” he said.

A spokesman for Giuliani said Sunday that the close ally of President Trump was in critical but stable condition with a publicly undisclosed ailment, prompting widespread concern and well wishes for his health.

Rudy Giuliani speaks at Madison Square Garden on Oct. 27, 2027.…