Vishnu's day. Fasting on Ekadashi grants 1000-yajna-equivalent merit. Recited as the Vaikuntha-door + the Pandavas' practice.
Why this date - the science
Lunar gravitation maximally affects body-fluids on Ashtami + Ekadashi + Purnima + Amavasya. Eating heavy food on these tithis worsens digestion + the disturbed lunar-influence. Modern double-blind studies on lunar-meal-disruption are inconclusive but the practice has 5000 years of empirical observation.
Ayurvedic wisdom
A 24-time-per-year regular fast = annual cumulative metabolic-reset. Charaka Samhita prescribes "langhana" (light eating / fasting) every fortnight for vata-pitta-kapha balance. Ekadashi food (only fruits + tubers + dairy + buckwheat) = exactly Charaka's prescription.
For the family & mind
The DISCIPLINE of skipping a meal regularly = builds will-power + body-mind awareness. Pre-modern science of intermittent fasting. The mantra-recitation on Ekadashi = mind-anchoring practice. Sleep is often better on Ekadashi (light dinner = easy digestion).
Go deeper - every Hindu festival aligns spiritual, astronomical, ayurvedic and emotional layers on the same day. Read the full festival science explainer, check today's Panchang, or see the full festival calendar.