Rama’s birth, placed at midday — so madhyahna decides it.
Ram Navami is Chaitra Shukla Navami — the Navami of the shukla fortnight of Chaitra. It is kept at midday, so the date is the day that tithi is running madhyahna, the middle of the daylight hours.
Computed from the Moon’s elongation with the Swiss Ephemeris, sidereal, Lahiri ayanamsa. A tithi begins and ends at a real minute rather than at midnight, which is why the moment above decides the day — and why the same tithi read at a different hour can name a different date.
Cast a free kundli, or ask any of 24 astrologers what this year actually holds for you. Billed by the minute, and only while somebody is answering.