Helios had few great cult centres in mainland Greece, where his worship stayed comparatively marginal, but on the island of Rhodes he was the chief god of the state, honoured with the athletic festival of the Halieia and with the vast bronze Colossus that stood at the harbour and was counted among the wonders of the ancient world. From the fifth century BCE onward Greek poets and thinkers increasingly identified him with Apollo, whose associations with light and prophecy drew the two together, and by the Roman period Sol and Phoebus Apollo were often treated as one. The distinction mattered to scholars of religion even in antiquity: Helios is the visible disc, older and more elemental, while Apollo is a fully personal Olympian onto whom his solar role was gradually grafted.
Rome's own solar cult reached its height centuries after Helios's heyday in Greece. In 274 CE the emperor Aurelian, drawing on Syrian sun-worship he had encountered while campaigning in the east, established Sol Invictus, the Unconquered Sun, as an official state cult with its own temple and priesthood at Rome, and the god's festival, the Natalis Solis Invicti, fell on 25 December, the winter solstice by the Julian calendar then in use. Because this solar festival coincided so closely with the emerging Christian celebration of Christ's nativity, historians of religion generally treat the Roman sun cult as part of the cultural backdrop against which the 25 December date for Christmas later became fixed in the fourth century, though the exact relationship between the two dates remains debated among scholars.