Why is the ninth month called September?
1869, hypothetical reconstruction of the tetragrammaton YHWH (see Jehovah), based on the assumption that the tetragrammaton is the imperfective of Hebrew verb hawah, earlier form of hayah "was," in the sense of "the one who is, the existing."
the God of the Israelites, his name being revealed to Moses as four Hebrew consonants (YHWH) called the tetragrammaton. After the Exile (6th century BC), and especially from the 3rd century BC on, Jews ceased to use the name Yahweh for two reasons. As Judaism became a universal religion through its proselytizing in the Greco-Roman world, the more common noun Elohim, meaning "god," tended to replace Yahweh to demonstrate the universal sovereignty of Israel's God over all others. At the same time, the divine name was increasingly regarded as too sacred to be uttered; it was thus replaced vocally in the synagogue ritual by the Hebrew word Adonai ("My Lord"), which was translated as Kyrios ("Lord") in the Septuagint, the Greek version of the Old Testament.