How to use a festival guide correctly
Start with the defining rule on the page: the tithi, paksha, lunar month, or solar ingress that marks the observance. That tells you why the festival falls when it does and keeps you from treating last year’s Gregorian date as a reusable shortcut.
Then open the Panchang for your own city. Sunrise, moonrise, and local timezone can shift a vrat or parv by a day, especially when the tithi changes near dawn or the observance depends on a specific window such as madhyahna, pradosh, or nishita kaal.
If you need a pooja window, do not stop at the date page. Move to Muhurat Finder or your local temple calendar for the final ritual timing, because a festival date and a ceremony muhurat answer related but different questions.