During Purnima, people observe a fast throughout the day. The Purnima fast begins early in the morning and ends by seeing the Moon. On the day of Full Moon or Poornima, people worship Lord Vishnu in various forms and Avatars. As per the Vedas, the change of Moon has certain effects on the human body like some behavioral change or change of moods. Hence, it is believed that if you observe the Purnima fast, there won’t be any such change.