xara asked: yes, but if, for example, if said new moon fell on dec. 21 at 1.20am, would you celebrate on the 20th or the 21st?

For religious purposes I reckon the day to start and end at sunset. The period from sunset to sunrise is the eve and the period from sunrise to sunset is the day. In xara's example the period of celebration (eve/day) would be from sunset the 21st to sunset the 22nd. Note that while the instant of equinox/solstice and the instant of new/full moon are the same throughout the world, the instant of sunset is very local. This means that the point at which the New Year starts sweeping across the Earth would change from year to year and not be fixed by arbitrary Greenwich based lines of longitude. Moreover, it would sweep across the Earth and not jump by time zones.