Cities regularly hold several major cultural events within the same few weeks and then remain quiet for months. The clustering is a result of constraints that all point at the same part of the calendar.
Weather narrows the outdoor window
Open-air events depend on conditions that are tolerable for audiences standing outside for hours, which limits the usable period in temperate climates.
Organisers avoid the edges of that window because a cancellation caused by weather destroys a budget that has already been spent.
The safe period is therefore short, and every outdoor event in a region is competing for the same handful of weekends within it. Cancellation insurance is available and expensive, which pushes organisers further toward the middle of the season.
School holidays determine who can attend
Family attendance depends on holiday periods, and those dates are set regionally and published years ahead.
Events aimed at families schedule directly into them, while events avoiding families sometimes schedule deliberately outside.
Because the holiday calendar is staggered across German states, neighbouring regions can find their festival seasons pushed into different weeks.
Technical suppliers are shared and finite
Staging, sound systems, lighting rigs and the crews who operate them are hired from a limited regional pool serving every event.
During peak weeks that pool is fully booked, and prices rise accordingly, which is a substantial line in any festival budget.
Some organisers move dates specifically to escape the competition for equipment, trading a better date for a workable production. Long-standing events hold their bookings year to year, which leaves newer ones taking whatever capacity is left.
Artist routing pulls events together
Performers tour in geographic sequences, and a festival is far more likely to secure an act already passing through the region.
That makes it advantageous to schedule when others nearby are scheduling, since the touring traffic is concentrated in those same weeks.
The result is a pull toward clustering that works against the pressure on suppliers, and organisers balance the two each year.
Concentration has real costs
Audiences must choose between overlapping events, and smaller ones lose attendance to larger neighbours competing for the same weekend.
Cities absorb concentrated demand on accommodation and transport, and then face quiet periods when venues and staff are underused.
Attempts to spread the calendar through winter and indoor programming address exactly this, and their success depends on whether audiences will travel when the weather does not invite it.