Almost every date in Hindu observance is given as a tithi rather than a calendar date. Ekadashi, Chaturthi, Amavasya, Purnima — these are tithis. Understanding what one actually is explains most of what people find confusing about the Hindu calendar, including why two households can observe the same festival on different days.
The definition, which is astronomical rather than traditional
A tithi is the time the Moon takes to gain twelve degrees of angular distance on the Sun.
That is the whole definition. At the new moon the Sun and Moon share the same celestial longitude, and the separation is zero. As the Moon moves ahead, that gap opens. Every twelve degrees of gap is one tithi. Since a full circuit is 360 degrees, a lunar month contains exactly thirty tithis.
Why a tithi is not a day
This is the part that matters, and it follows from the definition rather than from custom.
The Moon does not travel at a constant speed. Its orbit is an ellipse, so it moves faster when nearer the Earth and slower when further away. The gap between Sun and Moon therefore widens at an uneven rate, and a tithi — a fixed twelve degrees of that widening — takes a variable amount of time. In practice a tithi runs roughly nineteen to twenty-six hours.
Three consequences follow, and together they account for nearly every apparent inconsistency in festival dating:
- A tithi can begin and end at any clock time. It has no reason to align with sunrise or midnight.
- A tithi can be skipped, or repeated. If a tithi begins and ends entirely between two sunrises it is a kshaya tithi and that number is missing from the calendar. If a slow tithi spans two sunrises it is an adhika tithi and the number appears twice.
- Which day a festival falls on depends on a rule, not just on the tithi. Different traditions specify a different moment for the test — the tithi current at sunrise, or at moonrise, or at midday, or at the particular watch of the night the observance belongs to. Two panchangs applying different rules to identical astronomy will print different dates, and both are correct within their own convention.
The thirty tithis, and the two halves
The lunar month divides into two pakshas of fifteen tithis each.
Shukla paksha is the bright half, from the new moon to the full moon, as the lit portion grows. Krishna paksha is the dark half, from full moon back to new. Within each, the tithis are named in order: Pratipada, Dvitiya, Tritiya, Chaturthi, Panchami, Shashthi, Saptami, Ashtami, Navami, Dashami, Ekadashi, Dvadashi, Trayodashi, Chaturdashi — and then the fifteenth, which is Purnima (full moon) at the end of the bright half and Amavasya (new moon) at the end of the dark half.
Which tithis observances attach to, and why
A handful of tithis carry most of the ritual calendar, and the pattern is not arbitrary.
- Chaturthi, the fourth — associated with Ganesha.
- Ashtami, the eighth — a quarter-phase, associated with the Devi.
- Ekadashi, the eleventh — the principal fasting tithi of the Vaishnava calendar, observed in both pakshas.
- Trayodashi and Chaturdashi — associated with Shiva, and the Krishna paksha Chaturdashi is Maha Shivaratri.
- Purnima and Amavasya — the two extremes, and the two most widely observed.
The quarter, half and full points of the lunar cycle carry the weight. That is an observational calendar, built by people watching the sky rather than reading a table.
What AdiVeda does and does not publish
AdiVeda explains what a tithi is. It does not calculate or publish tithi dates, festival dates or muhurtas.
The reason is honesty about precision. A tithi at a given moment depends on the observer's location and on the ayanamsa and rule set in use, and a wrong date on a fasting day is a real failure, not a rounding error. Where a date matters to you, take it from a panchang you trust and that states its conventions.
Where this comes from
The tithi definition and the thirty-fold division are set out in the Surya Siddhanta and the classical jyotisha and calendrical literature, and are standard across every school. The observance associations are living tradition and vary by region and sampradaya; where they do, this page says so rather than picking one.
Related: what a panchang is, and how to read a nakshatra — the nakshatra being a second, independent way the same Moon is tracked.