The Stock Supply Day module adds a Week Days list on the Product Supplier form. This allow to restrict the supply week days for each supplier on each product. If no days are defined for a supplier a supplying may happens at any day of the week.
For example, those week days are taken into account when computing the planned date of stock moves created by the purchase (defined in the Purchase module) and by the scheduler that generates the Purchase Requests (defined in the Stock Supply module).