Mahavir Nirvana Utsav (Diwali) and the Hindu Diwali often fall on the same day because both are tied to the same lunar timing in Kartik. In Jainism, Mahavira’s nirvana is traditionally observed on Kartik Amavasya (the new moon night in Kartik), which is the same period when Hindus celebrate Diwali. So the coincidence is due to using the same Amavasya/tithi in the lunar calendar, not because of any cross‑religious calendar rule. Different Jain traditions give slightly different year counts, but the spiritual meaning—celebrating Moksha and the light of true knowledge—remains the same. You can read more here and here .