Hinduism follows the lunar calendar. Each month, therefore, has one full moon night or 'Purnima' and one full dark night or 'Amavasya' . Month to month, each of these days holds some special religious significance.
People usually fast every Purnima day, round the year. Some people take dips in holy rivers. Some others hold special puja or mantra japas on Purnima day.
Many people conduct the Satya Narayan Vrat, upvas, and puja on this day, round the year. Purnima is special as the rays of the moon is supposed to be cooling, calming and life giving.
Many people keep 'kheer' or rice pudding, under the rays of the moon-lit night. And next morning, before sunrise, all family members eat the pudding as 'prasad', with the intention that the moon rays has added Elixir, or Amrit to the kheer.