The Science of Hindu Calendar
The Hindu calendar is a lunisolar calendar that tracks both the sun and the moon. This is why a festival like Maha Shivaratri falls on a different day every year on the Gregorian calendar, but usually remains in the same lunar month (Phalguna).
Tithis and Timings
The date of a Hindu festival is determined by the Tithi (lunar day). However, because a Tithi can span across two Gregorian days, the festival is celebrated on the day when the Tithi is active during a specific auspicious time (like sunrise or twilight).