"I didn't know it was Thanksgiving until like midday. Until [the] afternoon," he told PEOPLE in an interview for the coming issue.
"When you're overseas, they don't celebrate it in England. And [Angelina Jolie] and some of the kids are overseas in Cambodia this week, working on the [The Maddox Jolie-Pitt Foundation] there," he said. "It completely escaped me until someone brought me some English pumpkin pie."
Even though Pitt, 48, went without the traditional turkey and stuffing this year, he didn't seem to mind, saying, "That's okay, last year we had a big meal and we don't normally miss things twice."
Still, Pitt was certainly into the spirit of the holiday. He put the entire contents of his wallet, $1,100, into the hands of a fundraiser collecting money for the Southampton General Hospital's neonatal unit, where a friend's son, Zachary Gallagher, had spent the first nine weeks of his life.
"The fundraiser had finished, and I got a phone call from my best friend, who said, 'We have had a small donation.' I thought she was messing about and was going to say, 'My dad's given you a fiver' or something, then she said, 'It's from Brad Pitt!' " Zachary's mother, Karley Gallagher, 23, tells PEOPLE.
"He said that he had seen the fundraiser mentioned in the local papers and seen my fliers and thought it was a really great cause and was just sorry he couldn't have been there," she adds "The silver lining is that Zachary is home and alive. ... To have Brad assist us is more than we could ever have wished for."
While Pitt, in New York Monday for a special screening of Killing Them Softly, plans to fly back to London on Tuesday to continue filming, the Jolie-Pitt bunch is already focused on the next holiday.
Maddox, 11, Pax, 8, Zahara, 7, Shiloh, 6, and 4-year-old twins Knox and Vivienne have already mailed letters to Santa from the village of Littlebourne in rural Kent, England.
Reporting by MARY GREEN and PHIL BOUCHER