Question

why do we see a complete round moon on the full moon day but no moon in the sky on the new moon day

Answer 1

As the Moon moves eastward away from the Sun in the sky, we see a bit more of the sunlit side of the Moon each night. ... We call this phase the full moon. It rises almost exactly as the Sun sets and sets just as the Sun rises the next day. The Moon has now completed one half of the lunar month.

Answer 2

we see a complete moon on full moon day because in this day, minn receive sunlight on its complete part and reflect this light at night.
But on new moon day, it doesn't get any light at all so it doesn't reflect any light and we see no moon on this day.