The provisional hold is what makes this trade unusual. A date is held without being booked, so it needs a deadline and an automatic release — otherwise an undecided enquiry ties up the best Saturday of the year.
Deposits and balances structure the whole year. With twelve to eighteen months between booking and event, staged payments are the only thing that brings a cancellation forward while the date can still be resold.
Layout, staffing and the catering order all depend on a number confirmed at the last minute. A cut-off written into the contract means planning happens once rather than three times.
Suppliers arrive on the day expecting access. Florist, band, caterer and their delivery times belong on the booking itself, because on the morning it is somebody who never read the email chain who has to let them in.