Purnima is the full-moon tithi that completes Shukla Paksha. The Moon is then 180° from the Sun and fully illuminated. Many of the most important Hindu festivals fall on Purnima: Guru Purnima, Raksha Bandhan, Sharad Purnima, Kartika Purnima, Holi and Buddha Purnima.
Each Purnima takes the name of its lunar month — Chaitra Purnima, Vaishakha Purnima and so on. In North India the Purnimanta calendar begins each month the day after Purnima, so the Purnima marks the end of a month there, while in the Amanta calendar it falls in the middle.
Purnima vrat and Satyanarayana puja are performed on the day Purnima prevails; when the tithi begins in the afternoon, the vrat may be kept on the previous day, so check the tithi start and end times listed for each date.