Question

why solar eclipse occurs only on a new moon day?

Answer 1

An eclipse  of the Sun can only occur at new moon when the moon passes between Earth and Sun. If the moon's shadow falls upon Earth's surface at that time, we see some portion of the Sun's disk covered or 'eclipsed' by the Moon.

Answer 2

Solar eclipse. ... During a partial solar eclipse (right), the Moon blocks only part of the Sun's disk (October 23, 2014). A solar eclipse occurs when a portion of the Earth is engulfed in a shadow cast by the Moon which fully or partially blocks sunlight. This occurs when the Sun, Moon and Earth are aligned.