Scheduling Workflow Notifications For Specific Days

We would like the option to schedule workflow notifications (i.e. invoices to approve) for certain days of the week. This would allow us to have our clients be notified of invoices to approve on days specified by them (i.e. Tuesday & Thursday), as well as allow us to turn off notification emails for internal users on weekends. The layout would be a simple grid of each day with a check mark to indicate whether an email notification should be sent that day.