From the Dictionary of Greek and Roman Biography and Mythology:
The worship of Adonis, which in later times was spread over nearly all the countries round the Mediterranean, was, as the story itself sufficiently indicates, of Asiatic, or more especially of Phoenician origin. (Lucian, de dea Syr. c. 6.) Thence it was transferred to Assyria, Egypt, Greece, and even to Italy, though of course with various modifications. In the Homeric poems no trace of it occurs, and the later Greek poets changed the original symbolic account of Adonis into a poetical story. In the Asiatic religions Aphrodite was the fructifying principle of nature, and Adonis appears to have reference to the death of nature in winter and its revival in spring -- hence he spends six months in the lower and six in the upper world. His death and his return to life were celebrated in annual festivals (Adônia) at Byblos, Alexandria in Egypt, Athens, and other places.