The key source of our information about Greek mythology is the poem called "Esiodou Theogonia", Hesiod's Theogony. As it has come down to us it appears to have been added to here and there is the usual question about whether Hesiod, defined as the author of "Esiodou Erga kai Emerai", Hesiod's Works and Day's, is also the Theogony. In any case it is poem from the times of Hesiod - a few years before 700 BCE. The Hesiod of Works and Days lived at Ascra near Thespiai in Boeotia in Greece but traveled as far away as Khalkis on Euboia.
A great deal of what has come down to us of the Greek mythology is an expansion of or a comment on the Theogony. But there is also a considerable amount of material independent of and sometimes contradictory to the Theogony. Much of this is preserved in much later sources.