On Aatham (8th tithi) and Chaudas (14th tithi) Jains generally try to keep extra saiyam (restraint). So, along with food rules, many elders also say: “Nakh–vāl na kaapvā” – don’t cut nails or hair. Main reasons:

1. Ahimsā (non‑violence) angle

- Cutting nails or hair usually means using blades, scissors, machines etc. - Tiny jīvs (micro‑organisms) on the skin, scalp, around nails can be harmed. - On normal days this basic himsā is tolerated as unavoidable (ārambhaj hiṁsā of a householder), but on Aatham–Chaudas we voluntarily reduce even such small himsā as much as possible. - Just like we avoid greens, fermented food, etc. on these tithis, we also avoid extra cutting / scraping of the body.

1. Day of tapa and shuddhi, not “beauty-care”

- Aatham and Chaudas are meant for upvās, ayambil, pratikraman, svādhyāy, samayik etc. - The bhāva (inner mood) should be of simplicity, detachment and repentance – not of decoration and grooming. - Hair‑cut, styling, manicure–pedicure, parlour‑type activities go in the opposite direction (sharir‑shṛngār, body‑decoration), so tradition says to avoid them on these spiritual days.

1. Control over rāg–dveṣ and body‑attachment

- “My look, my style, my appearance” easily increases moha and mān (attachment and ego). - By deciding, “Today I will not even do small grooming like cutting hair/nails,” we practice samyam and accept the body as it is for at least that day. - This helps slowly reduce deh‑abhimān (body‑identification).

1. Practical tradition

- Especially in earlier times, nail/hair cutting often caused small wounds, bleeding, infection etc. – more himsā to visible and invisible jīvs. - To keep the tithi as clean as possible in every way, households simply made a rule: do such things on other days, not on Aatham–Chaudas.

1. Both traditions (short note)

- Shwetāmbara: Very common house-rule on Aatham/Chaudas and during Paryushan/other important parvas; usually taught along with “no greens, no heavy outside work, more pratikraman.” ( jainknowledge.com) - Digambar: Emphasis is the same – increase tapa and dharma‑kriyā on special tithis; exact family‑rules (like hair/nail cutting) can vary, but the spirit of extra ahimsā and saiyam is common.

So, it is not that cutting nails or hair is absolutely forbidden in Jainism; it is a saiyam‑niyam for special tithis: on Aatham and Chaudas we try to avoid all unnecessary bodily actions that can cause even subtle himsā and that pull the mind towards body and beauty instead of dharma.

For a simple explanation of Aatham/Chaudas and tithi‑maryādā, you can read more here