Purnima and Amavasya
Purnima is the full-moon day that ends the bright half (Shukla paksha) of the lunar month; many keep the Satyanarayan vrat on it. Amavasya is the new-moon day that ends the dark half (Krishna paksha), traditionally used for remembering ancestors (pitru tarpan). Major festivals such as Holi, Raksha Bandhan and Guru Nanak Jayanti fall on a Purnima, and Diwali falls on an Amavasya.
Dates follow the panchang for New Delhi (IST), worked out from the positions of the Sun and Moon; tithi times are shown in IST. Some regions and traditions observe a festival a day earlier or later, so check with your family priest or local temple for rituals.