How festival dates are fixed by the panchang
Many Hindu festivals are set by the tithi, the nakshatra or a sankranti, so their dates come from the panchang rather than from a fixed calendar date.
What it is
A festival set by a tithi falls on the day when that tithi is present at the time the festival rule requires, often sunrise or midday. When the tithi starts or ends near that time, the festival date can differ between calendars.
How it is worked out or read
The site shows the tithi and its times in the panchang. It does not name festivals or set their dates, because the rule for each festival is tradition-specific.
Where it appears on this site
The tithi is in the panchang. The Ekadashi article gives one example of how the rule works.
The basic rule: the tithi at sunrise
Most Hindu festivals are tied to a tithi in a particular lunar month. The basic rule in most traditions is that the day on which the required tithi is current at sunrise, the udaya tithi, is the day of the festival. Because tithis do not line up with civil days, the festival's civil date moves from year to year, and in a year when the tithi begins after one sunrise and ends before the next, the almanac must decide which day to use.
Festivals that depend on a time of day
Several important festivals depend on the tithi at a particular time rather than at sunrise. Janmashtami is commonly fixed by the ashtami tithi at midnight, the time of Krishna's birth, with some traditions also requiring the Rohini nakshatra. Mahashivratri is fixed by chaturdashi at midnight. Diwali's Lakshmi puja is held when Amavasya prevails in the evening period called pradosh. Ram Navami looks to navami at midday. Holika Dahan is performed on the Purnima evening, avoiding the period called Bhadra. These rules explain why a festival is sometimes kept on a day when the almanac's sunrise tithi is different.
Different traditions, different dates
Communities can follow different rules. Smarta and Vaishnava traditions, for example, sometimes keep Janmashtami or Ekadashi on different days, because Vaishnava practice avoids an Ekadashi that is touched by the preceding Dashami at sunrise. Regions that follow amanta and purnimanta months give different month names to the same dark fortnight, though the festival date itself is the same. Solar festivals such as Makar Sankranti and Vishu depend on the Sun's entry into a sign rather than on a tithi.
Where to find festival dates
This site does not yet publish a festival list. For festival dates, the almanac of one's own community or temple remains the best reference, because it applies that community's rules. The panchang page here gives the tithi and nakshatra at sunrise and their end times for any date and city, which lets a reader understand why the almanac chose a particular day, or check whether a tithi prevails at midnight or in the evening.
Why the same festival differs between cities
A tithi ends at the same instant everywhere, but sunrise and midnight fall at different instants in different cities. A tithi that ends shortly after sunrise in the east of India may end before sunrise in the west, so the two regions can keep the same festival on different days. For families spread across countries, the difference can be larger still, which is why many families agree to follow one almanac.