The New Moon is the first phase in the lunar cycle, when the Moon lies between the Earth and the Sun and its lit face is turned away from us, so that its disc is dark and, in astronomical terms, invisible. In many lunar and lunisolar calendars the month begins here, either at the astronomical conjunction or, as in the Islamic reckoning, at the first sighting of the new crescent a day or so later. Traditionally the new moon is read as a time of beginnings, of planting the seed of an intention before its growth. It is also the only time a solar eclipse can occur, when the alignment is exact. These are the astronomical facts of the phase together with the meanings various traditions attach to it; the phases arise simply from the changing angle at which we see the sunlit half of the Moon.
The distinction between the new moon as astronomers define it, the moment of exact conjunction with the Sun, invisible even with instruments, and the young crescent that folk and religious calendars actually watch for is significant: the Babylonian calendar, and the Islamic calendar after it, both begin the month not at conjunction but at the first sighting of the crescent a day or so later, so that the civil new moon and the astronomical new moon can differ by a full day depending on latitude and horizon conditions. In Wiccan and related modern pagan practice, distinct from the older calendrical traditions, the dark of the moon before the first crescent is sometimes marked separately from the new moon proper, treated as a time of rest and quiet gestation rather than of active beginning, with the visible crescent reserved for the moment intentions are actually set in motion.
Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with The Oxford Companion to the Year: An Exploration of Calendar Customs and Time-ReckoningBonnie Blackburn and Leofranc Holford-Strevens, Wicca: A Guide for the Solitary PractitionerScott Cunningham