This is probably a Vinaya question, so I’d be glad if @Brahmali would be kind enough to respond, or anyone else of course who knows…
In the suttas the Uposatha is mentioned in several contexts - gathering, mythological, sila-practice. And there are three days given on which it can take place: ‘the 8th day’, ‘the 14th day’ and ‘the 15th day’ of the fortnight. Theoretically this should mean ‘the half-moon’, ‘the full and new moon’, and the day just before.
Yet when a specific moon-phase is mentioned in connection with the uposatha it’s always the full-moon (SN 22.85, MN 109, MN 110, MN 118, DN 2, DN 18, DN 19, Snp 3.12). Nowhere have I found an uposatha (or a Buddha discourse) that would take place at a ‘new moon’.
We only infer the ‘new moon uposatha’ from the ‘15th’ which should mean both 15ths, i.e. the full and new moon.
Now my question, is it possible that originally the uposatha only took place on full moons and was expanded only later to new, and then to half-moons?
Where in the Vinaya is the new moon explicitly stated (apart from ‘the 15th’)?
Of the three dates the most auspicious was (in later suttas) certainly the 15th (see e.g how the ‘treasures’ appear to the wheel-turning monarch on a 15th uposatha only (in SN 22.96, MN 129, DN 17) or how the ‘Four Great Kings’ watch over the 15th, and only their sons and ministers over the 8th and 14th.