Many significant festivals fall on a different date each year, sometimes moving by weeks. The cause is that the calendar setting the festival is not the one used for planning travel.
Lunar months do not fit a solar year
A cycle of the moon runs about twenty-nine and a half days, so twelve of them fall short of a solar year by roughly eleven days.
A purely lunar calendar therefore drifts backwards through the seasons, which is why some observances gradually move from summer towards winter over years.
Festivals fixed to such a calendar have a stable date within their own system, and it is the Gregorian date that appears to move.
Intercalation keeps other calendars aligned
Many traditions use a lunisolar system, adding an extra month periodically so that festivals stay within their intended season.
The insertion follows defined rules, but because it happens irregularly, the resulting Gregorian dates jump within a range rather than drifting steadily.
This produces the pattern where a festival falls several weeks later one year and returns to roughly its usual position the next.
Astronomical events anchor some observances
Solstices, equinoxes and specific lunar phases are used directly in several traditions, and these occur at a precise moment rather than on a nominal date.
Because that moment is measured in a particular time zone, the corresponding local date can differ between regions observing the same event.
Festivals dependent on a first sighting of the moon add further variation, since visibility depends on weather and location as well as astronomy.
Regional authorities can differ
Where observance depends on local determination, neighbouring countries occasionally mark the same festival on different days.
Travellers crossing a border during such a period may find businesses closed on one side and open on the other, with the shift announced only shortly beforehand.
Confirmation sometimes comes within a day or two of the event, which limits how far ahead a trip can be planned around it.
What this means for travel planning
Transport and accommodation around major festivals fill early, and the effect extends well beyond the festival days themselves because people travel to family.
Checking the date against a local source rather than a general calendar is worthwhile, particularly where the observance depends on sighting or local declaration.
Arriving in the days before rather than during the peak generally gives access to preparation and markets while avoiding the worst of the transport pressure.