Why is sour food (khaṭāī) forbidden in the Santoṣī Mātā Friday vrat?

the vrata-kathā makes it the central rule of the vow, so for 16 Fridays and at the udyāpana nothing sour is eaten or served.

The Santoṣī Mātā vrat, very popular in North and West India, is kept for 16 Fridays, usually for peace in the family and the fulfilment of a wish. Its offering is simple: roasted chana and jaggery (gur-chanā), a kalaśa of water, a lamp, and reading the kathā. The one rule everyone remembers is: no khaṭāī — no lemon, tamarind, curd, pickle or sour fruit — for the person keeping the vrat.

The reason given in the kathā is that the vow was broken when sour food was secretly given at the udyāpana, and the devotee’s troubles returned until it was done again properly. Beyond the story, the idea is santoṣa itself: a contented, simple, sweet offering.

At the udyāpana, eight boys are traditionally fed kheer-puri, and care is taken that no one in the house is served anything sour that day, nor given money that could buy it.

Prasāda: gur-chanā (jaggery and roasted gram) · Source: Santoṣī Mātā vrata-kathā

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