Here’s a simple Jain perspective on tithi (lunar-day) and food, kept plain and practical.

• There is no universal rule “what not to eat on every tithi.” Dietary practices by tithi vary by tradition (Digambar vs. Shwetambar) and by family or local temple customs. So, one tithi might have a practice in one community and a different one in another.

• A common foundational rule for many Jains (not strictly tied to a specific tithi) is to avoid root vegetables. So, onions, garlic, potatoes, carrots, beets, and similar roots are often avoided as a general practice, year-round.

• On certain tithis or during special Jain observances (for example during some months of Paryushan, or on specific upvas days), many followers observe a partial or full fast:

- Upvas (a day of fasting) can mean eating only fruit and dairy, or eating only one simple meal, or abstaining from solid food entirely, depending on the level of observance. - Some followers also choose to avoid certain foods (for example grains, lentils, or certain vegetables) on particular days, again depending on local guidance.

• Differences by tradition:

- Digambar and Shwetambar communities can differ on how strictly they interpret and apply tithi-based fasting and dietary rules. In many places both groups avoid root vegetables, but the exact fasting rules, what foods are permitted during upvas, and which tithis are observed as fast days can differ.

If you tell me which tradition you follow (Digambar or Shwetambar) and whether you’re asking about a specific temple or region, I can give you a clearer, more precise guideline for the tithi you have in mind.