What is Adhik Māsa (the extra month)?

an extra lunar month added about every three years to keep festivals in step with the seasons.

Because the lunar year is about 11 days shorter than the solar year, the gap builds up. Roughly every 32–33 months, there is a lunar month in which the Sun does not enter a new zodiac sign (no saṅkrānti). That month is called Adhik Māsa, “the extra month”, and the calendar repeats that month's name — for example, an Adhik Śrāvaṇa followed by the regular Śrāvaṇa.

Adhik Māsa is also called Mala Māsa and, more lovingly, Puruṣottama Māsa. Tradition says that when no deity wished to preside over this “ownerless” month, Viṣṇu (Puruṣottama) took it as his own. So while new beginnings such as weddings, gṛha-praveśa, mundan and upanayana are generally avoided, the month is considered excellent for devotion: reading the Bhāgavatam or Gītā, japa, fasting, dāna and pilgrimage.

Fixed festivals are celebrated in the regular month, not the adhik one. Annual śrāddha rules in Adhik Māsa are specific; ask your priest.

Source: Sūrya Siddhānta; Padma Purāṇa (Puruṣottama Māsa Māhātmya) · About every 32–33 months

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