Purnima is the word for full moon in Sanskrit. The day of Purnima is the day (Tithi) in each month when the full moon occurs, and marks the division in each month between the two lunar fortnights (, and the Moon are aligned exactly in a straight line, called a syzygy of the Sun–Earth–Moon system. Full moon is considered the third of the four primary phases of the Moon; the other three phases are new moon, first quarter moon, and third quarter moon. The full moon shows 100% illumination, causes high tides, and can concur with lunar eclipses.
Vat Purnima is celebrated on the full moon day of the month of Jyeshta. Women pray for their husbands by tying threads around a banyan tree (Vat) on this day. It honors Savitri, the legendary wife of Satyavan who escaped death for her husband's life. It is the chosen day for worshipping Yama deva
Guru Purnima, devotees offer puja (worship) to their Guru, on a full moon day. This is well known as Vyasa Purnima (Sanskrit. व्यास) after the birthday of the celebrated author of Mahabharata who is declared as the guru of all in the Shiva Purana.
Sharad Purnima or Kojagiri purnima, the Autumn Harvest Festival, on a full moon day.
Poornima, the day of birth, enlightenment and passing away of Gautama Buddha, on a full moon day.
Holi/Phalgun Purnima, the Spring Festival of Colours in Hinduism/Buddhism/Jainism, the full moon day in Phalgun.