Across many regions, each town holds its market on a particular day and never on the others. The rotation is a functioning system rather than a quirk of local habit.
Traders needed several towns to make a living
A single small town could not support a specialist trader daily. Serving five towns on five days produced enough combined custom to be viable.
The rotation therefore emerged from the trader's side of the transaction, not the customer's. Fixed days let the same sellers circulate on a predictable circuit.
This is why neighbouring towns almost never share a market day, and why the pattern remains stable across long periods.
Distance between towns was set by travel time
The circuit had to be walkable or drivable with a loaded cart between one evening and the next morning, which fixed the spacing of market towns.
Look at a map of traditional market towns in a region and the spacing is remarkably even, reflecting a day's transport rather than any modern logic.
Towns that fell outside that spacing rarely developed markets and often remained villages, which is visible in their size today.
Charters made the day a legal right
In many countries the right to hold a market was granted formally and specified the day. Holding a competing market nearby on the same day could be challenged.
That legal protection entrenched the rotation and gave towns an interest in defending their day, which is why some have kept the same one for centuries.
The charter often also fixed the location, which is why market squares sit where they do even when the town has grown around them.
The pattern shapes what is sold when
Livestock, produce and general goods historically ran on different cycles, with some towns holding a large periodic fair alongside the weekly market.
Those larger events survive in many places as annual fairs, and they retain a distinct character because their original function was different.
Produce markets tend to run earlier in the day and finish by early afternoon, since the rhythm was set by growers who had already worked a morning.
Visitors can use the rotation deliberately
A traveller basing themselves in a region can attend a different market each day by following the circuit rather than returning to the same town.
Local tourist offices publish the schedule, and it is one of the more reliable pieces of information available because the days rarely change.
Arriving in a market town on the wrong day gives an entirely different impression of the place, which is worth knowing before judging it.