The faithful began gathering at the weekend at the farm where a former nurse, Nancy Fowler, has for the past seven years relayed messages that she says are from the Virgin Mary.
The annual event, which is not sanctioned by the local Roman Catholic hierarchy, drew 30,000 believers last year, but the Rockdale County sheriff's department estimated between 100,000 and 150,000 people would turn out this year.
Ms Fowler, 47, has told the faithful this will be the last message the Virgin Mary will issue through her to the public - though she claims she has received hundreds of private messages from the Virgin Mary and Jesus since 1983.
The Virgin's alleged messages to the faithful through Ms Fowler began in 1991, a year after Ms Fowler said she had a vision of a farm where she was directed to go by Jesus Christ. She found the farm, in hills about 35 miles east of Atlanta, in Conyers, Georgia.
"Jesus appears to Nancy multiple times each day and continues to appear," her spokesman, Richard Pfundstein, said. "He gives her messages, some to share and some to keep private."
The messages from Mary initially came on the 13th of each month, but have come only annually since 1994. Ms Fowler said last year that Mary had told her this year's message would be the last.
Many Catholic leaders in the region have distanced themselves from Ms Fowler's claims. Archbishop James Lyke wrote to priests saying he doubted the authenticity of Ms Fowler's apparitions and urged them to discourage visits to the farm.
Rockdale County authorities have also expressed concern about the influx of visitors in the largely rural county with a population of just 60,000.
In 1991 the county health department asked Ms Fowler to post signs warning pilgrims about water from a well on her property. The well, which Ms Fowler said Jesus blessed in an appearance to her, tested positive for coliform bacteria.
In 1993, the Atlanta Constitution reported, the county threatened to declare large crowds at the farm a public nuisance. The complaint failed to deter the public, however, and in November of that year 80,000 people showed up.
Ms Fowler said that year that her monthly messages would stop and she would speak only once a year, on 13 October, the anniversary of a reported appearance by the Virgin Mary to three children in Fatima, Portugal, in 1917. Pope John Paul II has recognised the Fatima appearance as a factual event.
Caravans of buses began rolling into Georgia at the weekend for the final message. The authorities were more apprehensive about the traffic problems than the message itself. "It's going to be extremely bad, that's all I can tell you," Terry Stewart, a dispatcher at the Rockdale County sheriff's office, said.
Ms Fowler surprised the faithful on Monday with an impromptu appearance on her front porch. She took questions from her audience, but when one pilgrim asked what Jesus looked like, she said she did not know.
"I can't tell you that it was a visual thing," she said. "But it was a feeling of love. My heart was filled with love."
Ms Fowler has allowed herself to be subjected to scientific investigation. "When I first met Nancy, I thought she was crazy," said Ricardo Castanon, a neurophysiologist who was part of a team of scientists who studied Ms Fowler's encounters.
Mr Castanon, from La Paz, Bolivia, said he found Ms Fowler's brain produced delta waves similar to those experienced in deep sleep whenever she claimed to have had an apparition of Jesus Christ or Mary. Her heart rate, he said, dropped to minimal levels.
"When she sees Jesus, she has three or four heartbeats a minute," he said. "It is impossible for a person to have three or four heartbeats a minute. The brain only produces delta activity in deep sleep, and yet she was wide awake." - Reuters