Having two new moons occur in a single month only occurs every 32 months — and that’s exactly what’s happening on July 31.
Since there are 29.5 days between new moons, it sometimes happens that they both fall during the same month. In this case, the second of July’s new moons will be at the very end of the month on July 31.
A new moon is when the moon passes between planet Earth and the sun and is not visible from Earth. The moon appears dark to us Earthians because the side that’s lit by the sun at that time faces away from us.