I’ll answer in two parts:

1. What is “tithi” in Jainism?

• “Tithi” means a lunar day in the Jain (and Indian) calendar.
• Each month has 30 tithis, based on the moon’s position.
• Important tithis for Jains include:

- Aatham / Ashtami – 8th tithi - Chaudas / Chaturdashi – 14th tithi - Poonam / Purnima – full-moon day - Amas / Amavasya – no-moon day

On these days, many Jains:

• do extra samayik, pratikraman, tapa (fasting, upvas, etc.)
• keep food very simple and more ahimsak (less violence to living beings).

You can read a simple explanation about tithis and diet here

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2. Why avoid vegetables? (especially root / green vegetables on tithi)

In Jain Dharma, all food rules are based on one main principle: Ahimsa (non‑violence).

(a) Root vegetables – why many Jains avoid them

Examples: potato, onion, garlic, carrot, beet, radish, sweet potato, yam, ginger, fresh haldi, etc.

Reason:

1. Anant‑kay (innumerable lives)

- Many roots are considered sādhāraṇ vanaspati – one body containing countless souls. - When we pull them out, many lives are harmed at once, so himsa is higher.

1. Killing the whole plant

- To eat the root, the plant must be uprooted and fully destroyed.

1. More living beings in moist soil

- Digging and washing roots disturb many minute jivas in the soil and on the surface.

Because of these reasons, both Digambar and Shwetambar traditions teach avoiding root vegetables in normal Jain diet. Details may vary slightly region to region, but the main rule is the same. ( jainknowledge.com)

More explanation about root vegetables:

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(b) Why specially avoid some vegetables on “tithi” days

On special tithis like Aatham, Chaudas, Poonam, many Jains further restrict food:

Common practices:

• avoid green leafy vegetables (saag, palak, methi, dhania, etc.)
• some also avoid curd / fermented food
• keep food very simple: grains + dal/kadhi + dry pulses (kathol)

Main reasons: ( jainknowledge.com)

1. Life-force believed to be higher

On certain lunar days, it is believed that the pran (life-activity) in plants and small organisms is more active. So plucking and eating greens on those days is considered more himsa.

1. Extra care for Ahimsa

Green leafy vegetables usually have many tiny organisms attached. Avoiding them on tithi reduces subtle violence further.

1. Support for sadhana (spiritual practice)

Tithi is for more samayik, pratikraman, paath, tapa. Simple, limited food helps control taste, desire, laziness, and supports meditation.

This rule (no greens on tithi) is more common among Shwetambar Jains, especially as a 5‑tithi maryada or monthly discipline. Digambar Jains also keep stricter food on certain days, but exact lists differ according to acharya and local tradition.

You can see a clear, easy answer on tithi and green vegetables here:

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Short summary

• Tithi = special lunar day in Jain calendar (like Aatham, Chaudas, Poonam).
• On these days, Jains try to:

- reduce even very small himsa in food - increase sadhana (samayik, pratikraman, fasting, etc.)

• Vegetables avoided:

- Root vegetables: most of the time, because of anant‑kay and higher violence. - Green leafy vegetables: especially on certain tithis, to further reduce himsa.

All this is to slowly purify our bhav (inner attitude), make us more compassionate, and move us towards liberation.