Why is everything yellow in the Thursday (Bṛhaspati) vrat, and why do some avoid washing hair?

yellow is Bṛhaspati’s colour; the no-hair-washing rule is a regional folk custom, not a universal śāstric rule.

Thursday is the day of Bṛhaspati, the guru of the devas, and of Viṣṇu. Yellow stands for Guru’s wisdom and auspiciousness, so devotees wear yellow, offer chana dal with jaggery, yellow flowers, turmeric and bananas, and many worship a banana plant, which is linked with Viṣṇu. The single meal is often yellow too: besan, chana dal or kesar.

In parts of North India women avoid washing hair, washing clothes or cutting nails on Thursdays, based on the folk Bṛhaspati vrata-kathā, where such acts were said to drive away prosperity. Many families follow this lovingly; others do not. It is a custom of the kathā tradition, so follow your family’s way without worry either way.

The vrat is usually started on a śukla pakṣa Thursday (avoiding Pauṣa in some homes) and kept for 16 Thursdays or for a year.

Offerings: chana dal, jaggery, banana, turmeric · Source: Bṛhaspativāra vrata-kathā (folk tradition)

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