Your Vedic sun sign is the rashi the Sun occupied on your birth date by sidereal reckoning. The Sun stays about a month per sign and shifts at each sankranti — so the date is primary, and time decides only on sankranti days.
The Western sun sign uses the tropical zodiac, tied to the seasons; the Vedic uses the sidereal, tied to the stars. They currently differ by ~24° (the Lahiri ayanamsa) — which is why a Western 'Leo' usually turns out a Vedic Cancer.
Which sun sign do festivals follow?
Sankranti-based festivals (Makar Sankranti etc.) follow the sidereal Sun — exactly what this calculator gives.
What does the sun sign describe?
Self-radiance, the father's side, status and life-direction — but remember, Vedic prediction pivots on the Moon sign and lagna; the sun sign is a supporting angle.
What about a birth on a sankranti day?
The Sun changes sign that day — only the exact time settles the rashi; the result flags time-sensitivity on such days.
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