Ekadasi fasting days are calculated  using Vedic astronomy, focusing on Arunodaya Viddhi, meaning the Ekadashi tithi (lunar day) must be present during the auspicious pre-dawn Brahma Muhurta (about 96 mins before sunrise) for a "pure" fast; if it begins late, the fast moves to the next day (Dvadashi) as a Mahadvadashi, ensuring spiritual purity by avoiding conflict with the Dvadashi tithi during sunrise. 

Key Principles for Ekadashi Fasting Days Calculation: 

1. Sunrise-Based Calendar: Vedic days begin at sunrise, not midnight.
2. Arunodaya Viddhi (Brahma Muhurta): The critical factor is the tithi's presence during Brahma Muhurta, the period 1 hour 36 minutes before sunrise (or 2 muhurtabefore sunrise, each being 48 mins).
3. Suddha Ekadashi (Pure): If the Ekadashi tithi is present during Brahma Muhurta, it's a pure Ekadashi, observed on that day.
4. Mahadvadashi (Compounded): If the Ekadashi tithi starts after Brahma Muhurta (even just a few minutes after sunrise), it's considered "impure" and the fast is observed on the following Dvadashi, making it a Mahadvadashi.
5. Avoidance of Dvadashi Conflict: The goal is to observe the fast when it aligns perfectly with spiritual time (Brahma Muhurta) and avoids overlap with the Dvadashi during sunrise, which is spiritually inauspicious for Ekadashi. 

Example:

• If Ekadashi begins at 4 AM then last till 8 AM and sunrise is at 6 AM, it's observed on the same day (Suddha Ekadashi). 
• If Ekadashi begins at 6:10 AM and sunrise is at 6:00 AM and ekadasi tithi ends at 5:40 AM before next sunrise time which is 5:59AM, the fast moves to the next day (Dvadashi) because it wasn't present during Brahma Muhurta