Short answer

• Onion is a root (kand‑mool). Uprooting it kills the whole plant and disturbs countless tiny lives in the soil. This is more himsa, so it is avoided.
• On Jain discipline days like Satam/Aatham and Chaudas, many families keep extra-strong ahimsa. So even those who otherwise eat some items will avoid all greens and all roots that day. Onion naturally falls in this “do not eat” group.
• Onions are also pungent and excite the senses (rasa/kaṣāya), which goes against the calm and saiyam we try to keep on these tithis.

A little more detail

• Both Shwetambar and Digambar traditions advise avoiding kanda‑mool (onion, garlic, potato, etc.) because harvesting them destroys many lives (bahu‑jiva/nigoda). This is a general rule, not only for Satam/Chaudas.
• The specific “no‑green/no‑root” observance on certain lunar days can vary by region and family. Many say Aatham–Chaudas; some keep Satam–Chaudas. The intent is the same: reduce himsa and increase restraint on these pakshik (fortnightly) saiyam days.

For simple guides:

• Logic of not eating onion/garlic/potato:
• What we do not eat (Jain basics):