A festival is led either by its tithi or by its star. Ahnika Grantham says that of the avatara jayantis, Krishna's and Trivikrama's are star-led and every other is tithi-led. So Krishna Jayanti looks for Rohini, while Rama Navami is fixed by Navami alone, though Rama was born under Punarvasu. For the date itself, the printed almanac of your tradition decides. A book's rule explains a date; it does not overrule the almanac.
☕ Two clocks in the sky
The Moon gives two daily markers. The tithi counts its distance from the Sun. The nakshatra, or star, counts its place among the 27 star-groups. They run at slightly different speeds, so a tithi and a star meet on a different day each year.
A tithi-led festival is fixed by its tithi, and the star is incidental. A star-led festival is fixed by its star, with the tithi as a second test. Ahnika Grantham states the split for the avatara jayantis (Ahnika Grantham, Vol. 1 (Sri Ahobila Mutt), pp. 333–334).
Rama Navami shows the reasoning. Several older texts name Punarvasu, Rama's birth star. The book reasons past them: requiring the whole birth configuration would make the vrata almost impossible to keep, so Punarvasu is incidental and Navami alone decides (Ahnika Grantham, Vol. 1 (Sri Ahobila Mutt), pp. 330–333).
Krishna Jayanti is the other way. Kalaprakasika: the “combination of asterism Rohini with Ashtami, Navami or Dhasami without Vedhai signifies Jayanthi.” (Kalaprakasika (tr. N. P. Subramania Iyer, 1917), ch. XLI, p. 237). Here the star leads and the tithi may be one of three.
At a glance
- Tithi-led: Sri Rama Navami (Navami), Sri Narasimha Jayanti (Chaturdashi), and, by the book's rule, every avatara jayanti except two.
- Star-led: Krishna Jayanti in its Sri Jayanti form (Rohini), and Trivikrama's jayanti.
- Karthigai Deepam's general day is star-led (Krithikai). Kalaprakasika's own chapter calls the Full Moon its significator: “The Full-Moon is the fundamental significator of this festival” (Kalaprakasika (tr. N. P. Subramania Iyer, 1917), ch. XLIV, p. 239).
- The Smartha Krishna Janmashtami is tithi-led: Krishna Ashtami.
- What Nava does: Sri Rama Navami reads the tithi only, never a star. Krishna Janmashtami, for the Smartha profile, reads Krishna Ashtami. Pancharatra Sri Jayanti, for the Sri Vaishnava Pancharatra profile, reads Rohini or Mrigashirsha with Ashtami, Navami or Dasami. Karthigai Deepam reads the Krithikai star in the evening.
Frequently asked questions
Why is Rama Navami not kept on Punarvasu?
Ahnika Grantham treats Punarvasu as incidental. Requiring Rama’s full birth configuration would make the vrata almost unobservable, so the tithi, Navami, decides.
Why do Smarthas and Sri Vaishnavas keep Krishna’s birthday on different days?
The Smartha Janmashtami is tithi-led, by Krishna Ashtami. The Sri Vaishnava Sri Jayanti is star-led, by Rohini with a tithi. When Ashtami and Rohini fall on different days, the two observances part.
Is Karthigai Deepam led by the star or the Full Moon?
Its general day is the Krithikai evening. Kalaprakasika’s chapter makes the Full Moon its significator. The two fall close together: the chapter allows the Full Moon on a Bharani or Rohini day as well. When they part, each tradition follows its own almanac.
Deep dive: why the two drift apart
A tithi and a star meet on a different day each year.
🌙 Two cycles, one Moon
A tithi is 12° of the Moon's gain on the Sun; a star is 13°20′ of the Moon's own path. Over a lunar month the Moon passes through all 30 tithis and all 27 stars, so the pairing of a given tithi with a given star shifts from year to year.
A star-led festival therefore needs a fallback for the year its star and tithi miss each other, and the books give one.
🔗 Read first
This page builds on which time of day a festival checks, and on the nakshatra.