In Jain Dharma, there is no single universal dietary restriction fixed for every tithi.

Dietary observances depend on:

• which vrata or parva is being observed,
• Shvetambar or Digambar tradition,
• local custom, and
• the strength of one’s austerity practice.

General principle

On important tithis, Jains may observe one or more of these:

• Upavas — complete fast, usually without food or water
• Ekasana — one meal in a day
• Biyasana / Besana — two sittings of food
• Ayambil — plain, tasteless food without oil, ghee, spices, sugar, milk products, or many processed items
• Vigai tyag — avoiding rich items like milk, curd, ghee, oil, sweets
• Ratri bhojan tyag — avoiding food after sunset

Common tithis with special observance

Some tithis are especially important in Jain practice, for example:

• Ashtami — often observed with fasting or austerity by many devotees
• Chaturdashi — also commonly observed as a fast day
• Purnima and Amavasya — in some traditions, special worship and restraint
• Panchami / Dashami / Ekadashi / Trayodashi — may be important for specific vows, parvas, or temple traditions

Important note

The exact restriction for a tithi is not fixed in one common Jain rulebook for all Jains. For example, one community may do upavas, another may do ayambil, and another may simply avoid certain foods and eat before sunset.

Simple summary

So, the answer is:

• No separate fixed diet for every tithi
• Special tithis usually call for fasting or austerity
• The exact observance depends on tradition and vow

If you want, I can also give you a tithi-wise Jain fasting chart in a simple table.