How the festival day is counted
Two questions decide on which day a festival or an Ekādaśī fast falls: where the day is counted, and what the tithi is at sunrise and at Aruṇodaya.
1. Where: Mayapur or your location
Sunrise happens at different moments in different places, so the same tithi can fall on different civil days for devotees in Kyiv, Melbourne and Los Angeles. ISKCON has had two standards for this, and the app implements both:
| Standard | Reference location | |
|---|---|---|
| Before 1990 | Established by the Founder-Ācārya, His Divine Grace A. C. Bhaktivedānta Swāmī Prabhupāda | Śrī Māyāpur — one date for Vaishnava events all around the world |
| After 1990 | Proposed in 1990 | Your current location — sunrise where you are |
Choose the one your community follows in the app’s settings — read the GBC documents and how to choose. Dates for the current location option correspond to the ISKCON calendar GCAL 2011.
2. What: the tithi at sunrise
A tithi — a lunar day — starts and ends at any time of day or night, so it rarely coincides with a civil day. The rule is simple: the day gets the tithi that is in force at sunrise. That tithi names the day, and decides festivals and the days of Ekādaśī, Pūrṇimā and Amāvasyā.
Which sunrise? The one you chose in the settings: Celestial or Earth-sky horizon. The calendar uses the Celestial one.
3. Ekādaśī and Aruṇodaya
Ekādaśī needs one more check. A Vaishnava Ekādaśī must be śuddhā — pure, not mixed with Daśamī, the tenth tithi. So the calendar looks at a second moment, Aruṇodaya: 96 minutes before sunrise (two muhūrtas of 48 minutes).
- If Ekādaśī is already in force at Aruṇodaya, it is śuddhā — the fast is kept on this day.
- If at Aruṇodaya it is still Daśamī, the Ekādaśī is viddhā (mixed) — even though the tithi at sunrise is Ekādaśī, the fast moves to the next day, Dvādaśī.
In other words: Ekādaśī must begin earlier than 96 minutes before sunrise.
| Ekādaśī begins at | Tithi at sunrise | Tithi at Aruṇodaya | Result |
|---|---|---|---|
| 03:30 | Ekādaśī | Ekādaśī | śuddhā — fast on this day |
| 05:10 | Ekādaśī | Daśamī | viddhā — fast the next day |
| 06:20 | Daśamī | Daśamī | not an Ekādaśī day |
Example for sunrise at 06:00. In the second row the day formally has Ekādaśī at sunrise, yet the fast is not kept on it, because of Aruṇodaya.
4. Why accuracy matters here
Both checks — sunrise and Aruṇodaya — are sharp edges: when a tithi begins within a minute or two of one of them, a small timing error moves the observance to another day. That is why we validated the app against the JPL DE421 ephemeris.
Aruṇodaya is the fixed 96 minutes. Some traditions count it as four daṇḍas of a night whose length depends on season and latitude; the app does not do this.