The founding event of Thelema is unusually well dated because Crowley recorded it obsessively. In Cairo in March 1904 his wife Rose Kelly, who had no occult interest and no reason to perform, began producing communications; on 18 March she identified a particular funerary stele in the Bulaq museum, the object Thelemites call the Stele of Revealing, and on 20 March Crowley performed an invocation of Horus at her direction. On 8, 9 and 10 April 1904 he sat for one hour each day beginning at noon and wrote out the three chapters of Liber AL vel Legis, the Book of the Law, which he attributed to a discarnate intelligence named Aiwass.
What Crowley claimed about that text is a claim of dictation from a praeterhuman source. What is documented is that he wrote it, in those hours, in that room. Both statements are true and only the second is checkable, and the honest way to read him is to hold them apart.
The interesting part of the story is what he did next, which was nothing. He mislaid the manuscript, lost interest, and did not accept the book's authority for several years, behaviour hard to reconcile with the fabrication of a revelation, and one of the better arguments that whatever happened in Cairo, he was not simply lying about it. He spent the rest of his life on its consequences, defining magick as the science and art of causing change to occur in conformity with will, and building a corpus of ritual, poetry, commentary and instruction that no subsequent Western magical current has escaped.