Almost immediately after the public execution. The disciples had scarpered and only Magdalene, Salome and some other women had the guts to remain.
And I reckon that Celcius' account of Mary, her position as Temple-Virgin in a Greek Temple within hellenised Sepphoris (Zippori), the Roman retaking of Sepphoris from Judas BarEzekiah and her dalliance with a Roman soldier (Patronus?) which probably saved her life and freedom because all female residents were sold in to slavery, probably is the foundation upon which the myths were built. Nazareth is only about four miles to the south and was a hilltop working community in service to the city, hence Joseph's presence after the Romans left the area.
I reckon that could be the simple truth behind the legends and myths.
But it shatters the Abrahamic religions apart from Judaism. Blows 'em to the winds.
Why believe Celsus? (Note spelling.) He wrote in the 2nd century and was virulently opposed to Christianity, a common attitude among Romans in those days. Where did he get his information from after all those years when no one else seems to have known anything about it?
No one seems to know anything about a virgin birth until Matthew wrote about it around 80 AD or so. Where did he get his information? Paul and Mark would love to have had that kind of material to bolster their cases. Might it be that Matthew made it up? Such a story would very useful for establishing Jesus as the literal Son of God without getting into uncomfortable territory like Paul's pre-existing divine Jesus and its implication of polytheism. That would have distracted Matthew's Jewish oriented target audience away from the main message that Jesus was the Messiah.
People tend to believe the stories that support what they want to believe, despite basic questions like Where did the info come from?