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India uses one lunar calendar but two different conventions for where a month's boundary falls — at the new moon (Amanta, South India) or at the full moon (Purnimanta, North India). Both systems use the same 12 month names and observe the same festivals on the same days. The difference only surfaces during the Krishna (dark) fortnight, where the two systems assign different month names to the same 15 days.
New-moon to new-moon
The month starts on the day after Amavasya (new moon) with Shukla Pratipada, passes through Purnima (full moon) in the middle, and ends on the next Amavasya.
Full-moon to full-moon
The month starts on the day after Purnima (full moon) with Krishna Pratipada, passes through Amavasya (new moon) in the middle, and ends on the next Purnima.
Both systems agree completely during the Shukla paksha (bright fortnight, new moon → full moon). The month name is identical for those 15 days. The difference only appears during the Krishna paksha(dark fortnight, full moon → new moon), where the two systems assign different month names to the same 15 days.
The Vedic month (māsa) was originally defined by the moon's synodic cycle (~29.5 days). Both Amavasya and Purnima are natural “reset points” in this cycle — just 15 days apart. Different schools of jyotiṣa (Sūrya Siddhānta vs Kāśī tradition) chose different anchors for where the month restarts. Neither is wrong — they are two equally valid astronomical conventions.
South Indian Agama temples follow the Amanta system because Dakṣiṇa Bhārata dharmaśāstra texts, the Sūrya Siddhānta (pre-eminent in the South), and the practice of Tamil, Telugu, Kannada, and Marathi panchangam-makers all standardised on new-moon boundaries. Northern traditions (Kāśī panchang, Viṣṇudharmottara, Smārta usage in UP/Rajasthan) converged on Purnimanta, possibly because the Purnima is the most visibly dramatic lunar event — the bright full moon — making it a natural social “month divider” in a pre-electric era.
Both systems name the month by the nakshatra in which the Purnima moon falls. Chaitra ← Purnima near Chitra nakshatra. Vaishakha ← near Vishakha. Jyeshtha ← near Jyeshtha. This naming is identical in both systems — only the boundary of when the month starts and ends differs. The Purnima itself (and its nakshatra) is always the same calendar date.
For most people, the system choice only affects how you name a day during the dark fortnight. All major festivals — Diwali, Navratri, Holi, Ganesh Chaturthi, Janmashtami — fall on the same calendar date regardless of which system you use. The difference matters most for pandits when computing muhurta (the month name appears in the sankalpa mantra) and for scholarly/genealogical dates in Jataka horoscopes.
When no solar sankranti (sun entering a new rashi) falls within a lunar month, that month is declared Adhika (intercalary / leap). Both systems add the same intercalary month at the same time — the only difference is how the name is computed. In the Amanta system, the leap month takes the name of the month whose Amavasya has the same sun-rashi as the next Amavasya (i.e. no rashi change occurred). In the Purnimanta system, the Purnima naming rule applies similarly. For practical purposes, both panchangas call the leap month the same name (e.g. Adhika Jyeshtha in 2026). The nija (regular) month of the same name immediately follows. Observances and vratas should be done in the nija month unless a specific text prescribes Adhika masa observance.