You will notice that almost all fasting rhythms in the Vedic calendar (Ekadasi, Pradosham, Pūrṇimā) are tied to the Moon, while the Sun primarily dictates harvest festivals and external celebrations (like Sankranti). Why is this?

The Moon rules the Mind & Body

In traditional thought, the Moon (Chandra) governs fluids and the mind (Manas). Because the human body is mostly water, the Moon's gravitational pull physically affects our internal fluid dynamics and digestion. Because the mind is constantly fluctuating, it is tied to the Moon, which changes its phase every single day. Therefore, fasting—an internal discipline to detox the body and calm the mind—must align with the rapidly changing phases of the Moon.

The Sun rules the Seasons

The Sun (Surya) governs the soul and the physical seasons. It is steady, changing its zodiac sign only once a month. The solar calendar is used for external, environmental, and societal events. We celebrate the solar rhythm with outward expressions of gratitude (harvest festivals), but we fast on the lunar rhythm to master our internal environment.