Some people celebrate the purnima vrat today because the moon will be full during the day tomorrow. The fast needs to be completed before the moon is full and inches into the dark half of the month. Fasting before the moon is full is auspicious, according to Kabir Saheb, and it bestows benefits that cannot be obtained by fasting after the moon is full. Simply, we can say that the full moon represents fullness or completeness. Just so our fasting would be complete. Beyond the full moon represents a decrease as the moon increases in darkness and decreases in light. Our fasting should be full of light. Other people celebrate the fast on the day of the full moon, even if the moon is in the waning phase. They do not consider the time of the full moon.
Kabir Saheb also instructed us through Dharam Dass that the purnima vrat is the root of all fasting, and all the others are the branches. The above explanation would explain why the purnima vrat is the root. He said that Satpurush Parmatma gifted him with the purnima vrat, and he accepted it with humility. He then explained it to Dharam Dass, and through him to us.
Kabir Saheb also taught that the purnima vrat is from the beginning. We can understand this by realizing that the moon was manifesting its phases before we ever took birth on the earth. So it was in “operation” long before the dawn of consciousness in us. We can thus understand that the purnima vrat is from the beginning, because it is part of the cycles of manifestation in nature. And we are part of this eternal cycle.
It is important to observe the vrat with a clean body and, more importantly, with a clean mind. In the purnima katha, Kabir Saheb taught how to observe this vrat if you wish to derive the blessings that would flow from it. You know what a clean body and mind is. You also spend the day in prayer, meditation, satsangs, study of scripture, and noble thoughts and words. Love for God, people, animals, plants and all of nature is a key observance to take us further on our spiritual journey.