Why do Hindu festival dates change every year?
A Hindu festival is fixed to a point in the lunar month — a tithi within a paksha of a named month — not to a date in the Gregorian calendar. The lunar year is about eleven days shorter than the solar one, so each festival drifts backwards through the seasons until a leap month (adhika masa) resets it. A handful of festivals, Makar Sankranti and Baisakhi among them, are set by the Sun instead and barely move at all.
Why does my city’s date differ from India’s?
A tithi begins and ends at one instant everywhere on Earth — but the festival falls on whichever local day that instant covers. Sunrise in Houston is about eleven and a half hours behind sunrise in Delhi, so a tithi window that opens in a Delhi afternoon may not have opened yet by the previous Houston sunrise. That is how the same festival lands on different dates in India and the United States, and it is why a page that only shows Indian dates is no use abroad.