The term "new moon day" is used to refer to the phase of the moon known as the "new moon." It is called the new moon because, during this phase, the moon appears as a dark disk in the sky. The term "new" is used because it signifies the beginning of a new lunar cycle.
The moon goes through different phases as it orbits around the Earth. The new moon phase occurs when the moon is positioned between the Earth and the sun. From the Earth's perspective, the illuminated side of the moon is facing away from us, resulting in the moon appearing completely dark or invisible in the sky.
The term "new moon day" is used to describe this particular phase because it marks the start of a new lunar month. In many cultures and calendars, the lunar month is divided into phases based on the moon's appearance, with the new moon being the first phase. The following phases, such as the crescent moon, first quarter moon, full moon, and so on, are defined based on the changing appearance of the moon as it moves in its orbit.
So, "new moon day" is a commonly used term to indicate the specific phase of the moon where it appears as a dark disk and marks the beginning of a new lunar month.