Purnima

Purnima or Purnmasi is the full moon day of the Shukla paksh[i] (first fortnight) of the Hindu lunar calendar.
It is the brightest phase of the Moon.
It marks the completion of a lunar cycle.
It is the fifteenth tithi (date) of the bright fortnight.
It is observed as an auspicious day to observe fasting by the devotees and followers of Lord Vishnu.

There are twelve days of Purnima fasting in a Hindu lunar calendar.
Purnima fasting is observed either on Purnima day or on the day before Purnima i.e. on Chaturdashi (14^thlunar day).
The decision of Purnima fasting depends on the commencement or starting of Purnima tithi. Purnima fasting is observed on Chaturdashi if the Purnima Tithi starts during the Madhyahna period on the previous day.
Purnima fasting is observed from sunrise till sighting of the full moon in the evening/night.
Devotees offer the Satya NarayanPujaon this day.