Amavasai and Pournami, new moon and full moon in the Tamil calendar

The lunar month swings between two endpoints: Amavasai (அமாவாசை), when the Moon is invisible, and Pournami (பௌர்ணமி), when the Moon is full. Astronomically, Amavasai is the moment when the Moon and Sun have nearly the same ecliptic longitude (elongation ≈ 0°), and Pournami is when they stand opposite each other (elongation ≈ 180°). In the panchangam, Amavasai is the 30th and final tithi of krishna paksha (waning fortnight), and Pournami is the 15th and final tithi of shukla paksha (waxing fortnight).

Both days carry deep ritual significance. Amavasai is 'the ancestors' day', the day for ancestor rituals. Pournami is 'the temples' day', the day for worship and full-moon lamps. Together they form the lunar month's inhale and exhale.

Tarpanam: the ritual for ancestors

Amavasai is the regular day when Hindu families perform tarpanam (தர்ப்பணம்), a water ritual where one offers water, sesame seeds, and rice to deceased ancestors. The belief is that ancestors' souls (pitru) are closer to the earthly realm on new-moon day, and tarpanam eases their journey.

• Tarpanam is typically performed at sunrise or morning, facing south (the direction of the pitrus).
• Traditionally the men of the family are responsible, but in modern practice all adults perform it.
• Water mixed with black sesame (ellu) and rice is offered with specific mantras for three generations of ancestors.
• Thai Amavasai (January–February) is the most important tarpanam day of the year. Thousands travel to Rameswaram.
• Aadi Amavasai (July–August) is the second most important, with large gatherings at rivers and the sea.
• Mahalaya Amavasai (in Purattasi month, September–October) concludes the 15-day pitru paksha period.

Amavasai as an 'inauspicious' day

Amavasai is traditionally considered inauspicious for new beginnings, one avoids starting new projects, signing contracts, or holding weddings. The logic is that the Moon's total absence (darkness) symbolizes emptiness and closure. But Amavasai is not bad, it is a day for introspection, closure, and honoring those who came before.

Temple visits and full-moon rituals

Pournami is the most auspicious day for temple visits. Many temples hold special pujas, and devotees light full-moon lamps (vilakku) and perform pradakshina (circumambulation of the temple). Satyanarayana Puja, a Vishnu worship associated with prosperity, is traditionally held on Pournami.

• Chithirai Pournami (April–May): Full moon in the first Tamil month. A festive temple day.
• Vaikasi Visakam (May–June): When Pournami coincides with Vishakha nakshatra, Murugan's birthday.
• Aavani Avittam (August): Brahmins renew the sacred thread (Upanayanam), falls close to Pournami.
• Thai Pousam (January–February): Pournami near Pushya nakshatra, major Tamil temple festival.
• Guru Pournami (Aashada month): Homage to the spiritual teacher.
• Karthigai Deepam (November–December): Pournami in Karthigai month, lamp festival across Tamil Nadu.

Pournami and fasting

Pournami fasting is optional, it is not a mandatory viratham like Ekadasi. But many keep a light fast (fruit and milk) on Pournami and use the day for meditation and temple visits. Shiva devotees hold Pournami especially sacred, as Shiva wears the Moon in his hair.

In the panchangam, Amavasai and Pournami are determined by the Moon–Sun elongation, the angular difference between the Moon's and Sun's ecliptic longitude as seen from Earth.

Each tithi covers 12° of elongation. Amavasai (tithi 30) corresponds to elongation 348°–360°, Pournami (tithi 15) to 168°–180°. The moment of Amavasai/Pournami is when elongation crosses 360° or 180° respectively, but in the panchangam, it is the tithi at sunrise that counts.

Like all tithi-based observances, Amavasai and Pournami can fall on different dates in different cities. The reason is the same: the tithi at sunrise determines the day, and sunrise varies with a city's latitude, longitude, and time zone.

• Many Tamil families keep Amavasai free from major decisions, it is a day for silence, tarpanam, and remembrance.
• Pournami is the preferred day for temple visits, donation (annadanam), and starting positive habits.
• Sathabhishekam (80th birthday celebration) and certain pujas are planned for Pournami.
• Amavasai oil: many Tamils apply sesame oil (nalla ennai) to the body on Amavasai morning before bathing.
• Fishermen and farmers still use Amavasai/Pournami for planning: high tides at full moon, better catches at new moon.