New moon
The moment the Sun and Moon share the same ecliptic longitude — the start of the lunar cycle.
At a new moon the Moon sits between Earth and Sun with its lit side turned away, so there is no moon in the sky. Hence the habit of setting intentions at new moons: a moment that starts from nothing.
A lunation averages 29.53 days, so new moons come roughly every 29 and a half days, twelve or thirteen times a year. The sign it falls in is set by where the Sun is at that instant.
A new moon is not the same as "where the Moon is". The latter is the Moon's current position and aspects; the former is one specific Sun–Moon conjunction. They're different statements and worth keeping apart.