Cūḍākaraṇa — mundan, and what it marked
The popular claimThe first haircut leaves one lock, and the lock is the point.
Of the sixteen, this is the one most families still perform with a sense that it is a rite and not merely a haircut, and the texts repay that sense: the description is long, particular, and clear about what the rite is for.
What the sūtras give
Āśvalāyana and Pāraskara place it at the end of the first year or the third; Manu says the same. On the day, the child sits in the mother's lap, or the father's, facing east, near the fire. Warm water is brought and the father wets the hair, saying that he does so for long life, for splendour, for the sight of the Sun. Blades of darbha or kuśa grass are laid on the hair — the razor is not to touch the skin directly — and the father makes the first cuts with a copper or iron razor, addressing the razor as the tool with which Savitṛ shaved the beard of Varuṇa, asking it to cut without harm. Three cuts on the right, three on the left, three behind, each with its mantra; then the barber finishes.
But not entirely. The sūtras are explicit that a cūḍā — a tuft, the śikhā — is left, its number and position according to the family's gotra and school: one for some, three or five for others, at the crown or to the side. The cut hair is gathered on the grass and buried or placed in a cowshed, so that no one walks on it.
What the tuft was for
The rite is named for the tuft, not the shaving. The śikhā was the mark of a member of the Vedic household, kept through life and the last thing addressed at death, and Cūḍākaraṇa is the moment it is first given shape. The shaving around it is the means; the tuft is the end. A reader who thinks of mundan as "the removal of the birth hair" has the rite inverted — the sūtras never mention birth hair as impure, and the hair removed is treated with care rather than discarded.
Later Āyurvedic commentary adds a second layer: the fontanelle, the soft place at the crown, and the sense that the head should be shaved only once it has closed, which is around the first year. The sūtras' timing is consistent with that, though they do not give it as the reason.
Timing
The muhūrta treatises supply the day. Uttarāyaṇa — the Sun's northern half-year, Makara to Mithuna — is preferred; the child's birth month and birth nakṣatra are avoided; the mother should not be in the later months of another pregnancy; the tithi should not be the fourth, sixth, eighth, ninth, fourteenth or a new or full moon; the nakṣatra should be from the gentle or swift groups — Aśvinī, Mṛgaśira, Punarvasu, Puṣya, Hasta, Citrā, Svātī, Jyeṣṭhā, Śravaṇa, Dhaniṣṭhā, Śatabhiṣā, Revatī; the weekday not Tuesday or Saturday. This site's Best Date tool runs that table for a given child, including the personal layer from the birth nakṣatra.
The living practice
The mundan is kept nearly everywhere, and in two forms. The household form: a barber, a courtyard, the maternal uncle holding the child, the hair weighed against silver or wrapped in dough and set in a river. The temple form: the tonsure as an offering, at Tirumala above all, where tens of thousands of heads are shaved daily and the hair is the temple's, but also at Palani, at Vaishno Devi, at any shrine a family has a vow to. The temple form has no tuft; the head is shaved clean as a gift. It is not the sūtra's rite, and it is not less; it is a different act using the same occasion.
What this page does not say
It does not say that a child shaved on the wrong nakṣatra suffers for it, or that a head shaved clean at a temple has broken the rule about the tuft. The sūtra describes a household rite whose centre is the śikhā; the temple keeps a vow; the family keeps a first. This page is an account of the text and of what is done. The choice is the household's.
Source — Ashvalayana Grihyasutra 1.17 and Paraskara Grihyasutra 2.1 on Chudakarana; the Manusmriti 2.35 on the first or third year; Muhurta Chintamani on the timing conditions; the Sushruta Samhita on the fontanelle, which the rite's caution reflects
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