Observance of the Purnima Fasting is on the Purnima day or just one day before the Purnima day which is on the Chaturdasi Day. The start depends on the tithi of Purnima. The fasting which is done on the Chaturdasi depends on whether Purnima started on the Madhynana Period which occurs on the previous day. There is a belief that The Chaturdasi should not go beyond the Madhyana period because it pollutes Purnima tithi. The Chaturdasi should not be taken as the day for Purnima fasting even when the Purnima falls in the evening time.
Purnima is described as the full moon day in the Northern India. The case is different in South. Purnima is known as Pournami and the fasting is named as Pournami Vratam. The fasting is done from dawn till dusk. The Pournami fasting might have different timings in different locations. One should set the location according to the dates of Pournami Vratam.