What is pañcaka, and why do people avoid some work during it?

it is the roughly five-day stretch each month when the Moon crosses the last five nakṣatras; only a few specific activities are traditionally avoided.

Pañcaka begins when the Moon enters the second half of Dhaniṣṭhā and lasts through Śatabhiṣā, Pūrva Bhādrapadā, Uttara Bhādrapadā and Revatī — roughly while the Moon is in Kumbha and Mīna. It comes every month and lasts about five days.

The classical cautions are narrow: gathering firewood or grass, building a roof, making a cot, and starting a journey towards the south. If a death happens during pañcaka, many families perform a small pañcaka-śānti along with the last rites, following their priest.

It is not an inauspicious time in general. Weddings may still be avoided by some because of other factors, but routine work, office, purchases, studies and festivals go on as usual. Not every pañcaka is the same either — some traditions name them by weekday (for example Rog or Mṛtyu pañcaka) with different cautions.

Source: Muhūrta Cintāmaṇi; Garuḍa Purāṇa (pañcaka-śānti) · About 5 days each month

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