To avoid customers receiving an appointment confirmation because a work order cannot be closed.
You have to deal with it in practice. That sometimes we have to be smarter than a system. For example, if you have ordered parts for a customer that are on backorder. You want to keep the work order in view and cannot delete it.
If you set the work order date or plan date on a Sunday, no appointment confirmation will be charged and the customer will not receive unnecessary communication.
If you want to use a fixed date for this, you can also do so on 25 December, 31 December or 1 January.