Workflow Alerts are not sending on Sundays!

Workflow Alerts are not sending on Sundays!

We have spent several weeks troubleshooting a client instance of CRM with workflows that change fields and send alerts but do not appear to run correctly on Sundays only. They appear to run fine on Monday through Saturday, but on Sundays, they either don't run, or they don't send the email alerts. Customer is not happy.

How are they supposed to work?
  1. Leads have a date/time field of a future appointment,
  2. Appointments can be on Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, never Sunday,
  3. Workflow is supposed to run on the day before the date/time in the appointment field.
  • Workflow runs correctly on Monday for Tuesday appointments, Tuesday for Wednesday appointments, and so on.
  • But they never run on Sundays for Monday appointments. 
  • It may be that they run, they just don't send out the email alerts on Sundays.