In Shwetambar Jain tradition, “Ek Tithi” is a niyam (vow of restraint) about when you take food again.

• Meaning: You eat only after one lunar day (one tithi) has fully changed since your last meal. For example, if you ate during Pratipada (1st tithi), your next meal is only after Dwitiya (2nd tithi) begins. Because tithis vary in length, this often becomes more than 24 hours.
• How it’s observed: Take proper pachchakkhan (vow), keep full care of ahimsa and discipline, usually have only boiled, filtered water in the allowed daylight hours as per your maryada, do samayik/pratikraman, and do parna correctly.
• Purpose: To increase saiyam (self‑restraint), support samvar–nirjara (stopping and shedding karmas), and build regularity in sadhana.
• Tradition note: The name and popular use “Ek Tithi” is common among Shwetambar lay Jains. Digambar Jains also keep time‑based food vows, but the exact term/practice may differ by sangh. Always follow the maryada taught by your guru/sangh.

For a simple explanation of what a “tithi” is, see this reference from JainKnowledge: For related daily-time rules (like porsi/sadh‑porsi), see: