Here's why:

* The lunar cycle: The moon's phases are caused by the changing angle at which we see the sunlit portion of the moon as it orbits Earth. This cycle takes approximately 29.5 days to complete.

* Moon phases in a cycle: The entire cycle includes eight main phases: New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, and Waning Crescent. Each phase lasts roughly 3-4 days.

Therefore, we can observe each specific phase roughly once every 29.5 days.