Why do different families celebrate the same festival on different days?

because a tithi can span two days, and traditions differ on which moment of the day decides — sunrise, noon, evening or midnight.

The general rule is udaya tithi: the tithi present at sunrise governs the whole day. But a tithi often begins in the middle of one day and ends in the middle of the next, and the festival texts say each observance has its own key time. Gaṇeśa Caturthī and Rāma Navamī look to midday; Dīpāvalī Lakṣmī Pūjā looks to the evening (pradoṣa); Janmāṣṭamī and Mahāśivarātri look to midnight (niśītha).

So two careful priests may pick different days depending on which rule they give priority to. Sampradāya also matters: Smārta and Vaiṣṇava households often keep Ekādaśī or Janmāṣṭamī on different days because Vaiṣṇavas avoid a tithi “touched” by the previous one. And because sunrise time differs by city, the answer can even change between Delhi and Chennai.

None of this makes one family right and the other wrong. Follow your family's pañcāṅga or priest, and celebrate with a peaceful heart.

Source: Nirṇayasindhu; Dharmasindhu · Smārta vs Vaiṣṇava Ekādaśī

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