The 37-year-old actress has chosen the top secret date with some spiritual help, reported National Enquirer online.
"Angie has used Buddhist astrology in the past to help her pick the best or most auspicious days for important moments in her life. She's relied on the Buddhist calendar to plan medical procedures and vacations for herself, Brad and the kids. She's even used it for picking the dates of her movie openings," she said.
"That's when Angie was completely converted to Buddhist astrology and she's not taking that chance with Brad. Angie is convinced the date of their union will guarantee their future happiness," the source added.
According to friends, Pitt with whom she raises six children with, is happy to go along with whatever Jolie thinks and says she can take her time with choosing the date.