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Understanding Udaya Tithi, Madhyahna, Pradosh, Nishita & other Kala rules that determine festival dates
A tithi (lunar day) averages about 23 hours 37 minutes — shorter than a 24-hour solar day. This means a tithi can start during one sunrise-to-sunrise day and end during the next. When two consecutive solar days both contain the same tithi, which day do you observe the festival? This is the central question that the Kala-Vyapti (time-prevalence) rules answer.
The default rule for most festivals. Whichever tithi prevails at sunrise determines the day's tithi identity. If Chaturthi is running at sunrise on Tuesday, then Tuesday is treated as Chaturthi for ritual purposes — even if Chaturthi started the previous afternoon.
Festivals using this rule: Holi, Hanuman Jayanti, Govardhan Puja, Bhai Dooj
Classical source: Surya Siddhanta
The middle 1/5th of daytime (roughly 10:45 AM to 1:30 PM, varying with location and season). Used when the deity's birth or the key mythological event occurred at midday. The day on which the tithi is active during the Madhyahna window is chosen.
Festivals: Ram Navami (Rama born at noon), Ganesh Chaturthi (Ganesha born at midday), Akshaya Tritiya, Hartalika Teej