Purnima is the full moon day, the fifteenth Tithi of Shukla Paksha. It occurs once every lunar month (approximately every 29.5 days) and is considered one of the most auspicious days in the Hindu calendar. Many major festivals like Guru Purnima, Sharad Purnima, and Holi fall on Purnima. Fasting and charity on Purnima are believed to bring spiritual merit. The full moon also marks the boundary between months in the Purnimant calendar system.