What are Amānta and Pūrṇimānta months?

two ways of ending the lunar month — on the new moon (Amānta) or on the full moon (Pūrṇimānta).

In the Amānta system, used widely in Maharashtra, Gujarat, Karnataka, Andhra Pradesh and Telangana, a month runs from the day after one new moon to the next new moon. In the Pūrṇimānta system, common in North India — Uttar Pradesh, Bihar, Madhya Pradesh, Rajasthan and others — a month runs from full moon to full moon.

The bright half (śukla pakṣa) has the same month name in both systems. Only the dark half (kṛṣṇa pakṣa) gets a different name: what a North Indian calendar calls Bhādrapada Kṛṣṇa Aṣṭamī (Janmāṣṭamī), a Maharashtrian calendar calls Śrāvaṇa Kṛṣṇa Aṣṭamī. It is the very same day; only the label differs.

So when two calendars seem to disagree on a kṛṣṇa pakṣa festival's month, check which system each uses. Tamil Nadu, Kerala, Bengal and Odisha also use solar months for many purposes, which adds another layer of naming.

Source: pañcāṅga practice · Only kṛṣṇa pakṣa names differ

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