Introduction:
The day when we cannot see the moon in the sky is called a "New Moon Day." It is an important concept in astronomy and occurs approximately once a month.

Explanation:
New Moon:
- A new moon occurs when the moon is positioned between the Earth and the Sun, with the side of the moon facing the Earth being completely dark.
- During this phase, the illuminated side of the moon is not visible from Earth because the sunlight is only reaching the side facing away from us.
- As a result, the moon appears to blend in with the dark night sky, making it invisible to the naked eye.

Phases of the Moon:
- The moon goes through different phases as it orbits around the Earth.
- The four main phases of the moon are the new moon, first quarter, full moon, and third quarter.
- These phases occur due to the changing positions of the moon, Earth, and Sun.

Full Moon Day:
- A full moon occurs when the moon is on the opposite side of the Earth from the Sun, and its entire illuminated side is visible from Earth.
- On a full moon day, the moon appears as a complete circle in the sky.
- This phase is often associated with cultural and religious festivals in various parts of the world.

Crescent Moon Day and First Quarter Moon Day:
- A crescent moon occurs when only a small part of the illuminated side is visible, appearing as a thin sliver or crescent shape.
- The first quarter moon occurs when half of the illuminated side is visible, resembling a semi-circle.
- These phases are different from a new moon day because they involve some visibility of the illuminated side.

Conclusion:
In summary, the day when we cannot see the moon in the sky is called a "New Moon Day." It occurs when the moon is positioned between the Earth and the Sun, causing the illuminated side to face away from us. On this day, the moon appears invisible as it blends in with the dark night sky.