The Gregorian calendar — used as the global civil standard — is a purely solar system. It divides Earth's 365.25-day orbital period into 12 months of fixed length, with a leap year every 4 years. It tracks only one thing: where Earth is in its orbit around the Sun. Dates are fixed, predictable, and universal.
The Panchang (Panchangam) is a luni-solar system. It simultaneously tracks the Moon's position relative to the Sun (for Tithi and Nakshatra), the combined position of both (for Yoga), and the weekday's planetary ruler (Vara). A Panchang entry for a single day contains far more information than a calendar date — it describes the astronomical quality of the day.
This structural difference explains why Hindu festival dates shift each year in the Gregorian calendar. Holi, Diwali, and Navratri are set by specific Tithi-Nakshatra combinations in the lunar month. The lunar year is approximately 354 days — 11 days shorter than the solar year. Each year, the festivals drift about 11 days earlier in the Gregorian calendar, until an intercalary month (Adhika Masa) is added every 2–3 years to resynchronise.
A few Hindu festivals are actually solar-fixed and fall on the same Gregorian date every year: Makar Sankranti (the Sun entering Capricorn — January 14/15), Pongal (the Tamil solar new year, January 14/15), and related regional harvest festivals. These use the solar calendar and do not drift.
In daily life, Indians use both systems in parallel. Civil dates (office, school, law) use the Gregorian calendar. Auspicious timing, festival planning, and ritual scheduling use the Panchang. A wedding invitation in Tamil Nadu will typically show both the Gregorian date and the Tamil Panchangam details (Tithi, Nakshatra, and Yogam) for the ceremony.