Although Ps. 104:19 is often interpreted as taking sides in a calendrical conflict by supporting the exclusivity of the lunar calendar with regard to fixing the religious festivals, it is proved here that this is not the case. In the present article an interpretation of this verse is carried out with recourse to new studies on biblical parallelism, taking in account the evidence from the Amarna hymn to the Sun as well as the Jewish Medieval exegesis, notably Ibn-Ezra. The term mo'ed is understood here as meaning 'distinct phase' rather than 'festival'. Thus Ps. 104:19 refers to the lunar phases as a feature of the natural world without endorsing a specific calendrical statement. Ben-Sirah used Ps. 104:19 as a point of departure for further reflection, invoking the term mo'ed when referring to the moon. While Sir. 50:6 retains the older biblical meaning of mo'ed, in 43:7 the author uses this term in the sense of 'festival' and by this states his opinion that the moon is the exclusive marker of the cultic calendar. While calendar conflict (solar vs. lunar) was not in action in biblical religion, it became a debated issue in the second century BCE, with Ben-Sirah constituting a primary example.