Many of the festivals worth planning a trip around fall on a different date each year. The movement follows rules that are consistent internally but do not align with the civil calendar.
Lunar months drift against the solar year
A purely lunar calendar counts twelve cycles of the moon, which totals roughly eleven days fewer than a solar year. Its months therefore migrate backwards through the seasons.
Observances tied to such a calendar move earlier in the Gregorian year each time, completing a full circuit over roughly three decades.
This is why a given festival can fall in high summer for one generation of travellers and in midwinter for another, without anything about the observance changing.
Lunisolar systems correct with an extra month
Other traditions keep lunar months but insert an additional month periodically so the calendar stays anchored to the seasons. The correction is applied on a defined cycle.
The result is a festival that stays within a season but swings across several weeks of the civil calendar, sometimes by a month between consecutive years.
Because the intercalation follows regional rules, the same festival can be observed on different days in different countries that share the tradition.
Some dates depend on observation
A number of calendars begin a month on the sighting of the new crescent rather than on calculation, which means the start can be confirmed only a day or two ahead.
Authorities in different places may declare on different evenings depending on weather and the criteria they apply, so neighbouring countries can differ by a day.
For a traveller this makes the surrounding week the plannable unit rather than a specific date, particularly for closures and transport demand.
Movable feasts follow a computed anchor
Several Christian observances are calculated from a spring full moon rule, and a chain of other dates is fixed relative to that anchor once it is determined.
Churches using different underlying calendars compute the anchor differently, which is why the same feast can be observed weeks apart in different traditions.
Once the anchor is set, everything downstream moves with it, so an entire cluster of holidays shifts together rather than independently.
Agricultural and civic timing
Harvest festivals, transhumance events and vintage celebrations are set against what the land is doing, so organisers confirm dates only when the season is readable.
Civic festivals are often pinned to a weekend rather than a date, meaning they slide by up to six days and occasionally jump a week when the calendar is awkward.
The practical consequence is that accommodation should be booked against a confirmed local announcement rather than against last year's dates.