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Why the same tithi can fall on two calendar days – and how the correct observation day is determined
A tithi lasts ~23 hours 37 minutes, which rarely aligns with a 24-hour solar day. When a tithi spans two calendar days, the question arises: on which day should the festival be observed? The answer depends on the Kala (time window) associated with each festival.
India uses two month-counting systems. In Amant (South India, Gujarat), the month ends at Amavasya (new moon). In Purnimant (North India), the month ends at Purnima (full moon). During Krishna Paksha (after full moon), the Purnimant month is one ahead of Amant. Diwali is on Kartik Amavasya in Amant – and also Kartik Amavasya in Purnimant (because both systems align during Krishna Paksha). The divergence appears for Shukla Paksha festivals.
Example: Holi is on Purnima. In Amant it is "Phalguna Purnima." In Purnimant it is also "Phalguna Purnima" (since Purnima ends the month, it is the last day of Phalguna). But discrepancies appear when Krishna Paksha festivals are referenced across systems. Our app uses Amant month names for all festival definitions (matching mainstream reference panchangs).
Default rule. The tithi prevailing at sunrise determines the day. Used for: Holi, Hanuman Jayanti, Dussehra, Govardhan Puja, Bhai Dooj.
Middle 1/5th of daytime (~10:45 AM – 1:30 PM). Used when the deity was born at midday. Festivals: Ram Navami, Ganesh Chaturthi, Akshaya Tritiya, Hartalika Teej.
Sunset to ~96 minutes after (4 ghatis). For lamp-lighting and evening worship. Festivals: Diwali (Lakshmi Puja), Dhanteras, Karwa Chauth.
The 8th muhurta of the night (~11:40 PM – 12:28 AM). Deities associated with the deep night. Festivals: Janmashtami (Krishna born at midnight), Maha Shivaratri, Narak Chaturdashi.
4 ghatis (~96 min) before sunrise. For pre-dawn ritual baths and purification. Used for: Narak Chaturdashi (Abhyang Snan), Chhath Puja.
Dwi-tithi (Spanning): When the same tithi is present at two consecutive sunrises – the tithi is long enough to span two days. General rule: for Ekadashi, choose the second day (longer fast); for all other tithis, choose the first day.
Kshaya Tithi (Compressed): When a tithi completes entirely between one sunrise and the next – it is not present at any sunrise. This is rare (~2% of days). The festival is observed on the previous day.
Adhika Tithi: When an extra tithi is inserted – the same numbered tithi occurs twice. The festival is observed on the second (Nija) occurrence, not the first (Adhika).
For Ekadashi fasting, Smartha and Vaishnava traditions follow different rules. The Smartha tradition follows Udaya Tithi (tithi at sunrise). The Vaishnava tradition adds an extra restriction: Ekadashi must not have "Vedha" (contamination) from Dashami – if Dashami lingers past sunrise, the Ekadashi fast shifts to the next day (Dwadashi). This is why Smartha and Vaishnava Ekadashis sometimes fall on different days.