Short answer: festival days are fixed by lunar days (tithis), and the usual rule is that a festival is kept on the civil day on which its tithi is current at local sunrise. Sydney's sunrise is 4–6 hours ahead of Kathmandu's, so when a tithi ends in the gap between the two sunrises, Australia and Nepal keep the same festival on different civil days. Most years most festivals coincide; a few do not, and both dates are "right" for where they are kept.
What a tithi is
A tithi is one-thirtieth of the Moon's cycle: the time it takes the Moon to gain 12° on the Sun. Because the Moon's speed varies, a tithi lasts anywhere from about 20 to 26 hours, and it starts and ends at astronomical instants that have nothing to do with midnight or sunrise. A civil day therefore usually contains the end of one tithi and the start of the next; occasionally a short tithi begins and ends between two sunrises (a kshaya tithi) and never "owns" a day.
The udaya rule
To assign a festival to a civil day, the panchang asks which tithi is current at sunrise — the udaya tithi. Kathmandu and Sydney have different sunrise instants, so if a tithi ends between Kathmandu sunrise (say 6:00 am Nepal time, 10:15 am Sydney time) and Sydney sunrise the next morning, the two places disagree by a day. That is the whole mechanism; there is no dispute about the sky, only about which sunrise you stand under.
Festivals that use other rules
- Diwali / Lakshmi Puja is kept on the evening the Amavasya covers pradosh (the hour after sunset), because the ritual is an evening one.
- Maha Shivaratri and Krishna Janmashtami are kept on the night the tithi covers local midnight (nishita).
- Makar Sankranti / Maghe Sankranti and Nepali New Year (Baisakh 1) are solar: the day the Sun enters sidereal Makara or Mesha.
- Ekadashi fasting has its own refinements — when Ekadashi begins before sunrise but Dashami was still running at dawn, some traditions move the fast a day later.
Amanta and purnimanta — why the month name changes but the day does not
South India and Nepal's astronomical panchangs count a lunar month from new moon to new moon (amanta); North India counts full moon to full moon (purnimanta). The dark fortnight before Diwali is "Ashvina Krishna" in one system and "Kartika Krishna" in the other, but the Amavasya itself is the same night. When two sources seem to name Diwali's month differently, they usually agree on the date.
How diaspora families decide
There are two honest answers. Keep the local day — the tithi as it stands at your own sunrise, which is what the tradition prescribes for wherever you live; or keep the home day — so that tika, phone calls and rituals line up with parents in Nepal or India. Many households do both: the puja on the local day, the family call on the home day. What is worth avoiding is a third thing — the date printed on a calendar made for another hemisphere with no note of which rule it used.
Our calendar for Australian and NZ cities shows, for every festival, the local date, the tithi's start and end in local time, the rule used, and the Kathmandu date where it differs, so the choice is yours to make with the facts in front of you.
Related
Dashain 2026 in Australia · Tihar 2026 in Australia · Today's Moon and nakshatra