Short answer
Each festival follows its own rule, so the date can change with your city. Diwali's Lakshmi Puja falls on the Amavasya (new-Moon lunar day) that runs through the evening, which is 8 November in 2026. Navaratri begins on the first lunar day of the bright fortnight of Ashvin. Pongal is solar: it follows the Sun entering sidereal Makara (Capricorn). The Festivals tool shows your city's dates.
How ChiChakra reads this
Lunar festivals are fixed by the tithi (lunar day), and most use the tithi running at local sunrise; Lakshmi Puja looks at the evening after sunset instead. Solar festivals such as Pongal follow the moment the Sun changes sign. ChiChakra applies each rule to your own city's sunrise and sunset.
Why India and the UK or US can differ
A tithi begins at the same instant everywhere, but that instant shows a different clock time in each time zone. If a tithi begins at 8 a.m. in Chennai, it misses sunrise there, so India observes it the next day. In London that same moment is before dawn, so the UK can observe it a day earlier.
Pongal works the same way with the Sun. In the usual Tamil rule, if the Sun enters Makara before local sunset the new month starts that day; if after, it starts the next day. A few hours' difference in sunset can move the date abroad.
Always choose the city where you will celebrate, not where your family lives. If your local temple lists a different day, ask which rule it follows and go with your community.
Good to know
North and South Indian calendars sometimes give the same lunar month different names, so a festival may appear under another month name without its date changing.