Two calendars have to agree. Selling a four-night programme means selling rooms and treatment slots at once, and a guest who arrives to find the mud bath fully booked has been sold something that did not exist.
The treatment rooms fill before the bedrooms. A spa with sixty rooms and eight treatment cabins is limited by the cabins, so occupancy figures alone flatter a week that was actually impossible to deliver.
Clinical information needs different handling. Conditions, contraindications and a doctor's notes are health data, and storing them alongside a booking without restricting access is a problem waiting to be noticed.
The empty weeks are predictable. Knowing which periods historically fill and which do not is what lets a spa aim its medical programmes and longer stays at precisely the months that need them.