You can do something like this:
1. In design view, get the form's properties and go to the Events tab. In the Current event, double-click the blank line. You'll see Event Procedure. Click here, then click the Builder (...) button to go to the code window. You'll need to put in code like this --
Code:
Select Case Weekday([YourDateField])
Case 1
[YourDateField].BackColor=255 'red
Case 2
[YourDateField].BackColor=65536 'yellow, I think
'keep going for the other 5 weekdays
End Select
To work out what numbers to use, open any form and select a range of background colours from the palette. After each one, check the BackColor property and write it down.
By putting the code into the Form_Current event, you get the formatting to apply every time you go a to new record. You'll need to ensure that the BackStyle property for the control is Solid. By default it's Transparent, and no colour change will be visible.
If you need to fire the change when a field changes, leave the Form_Current code as it is. But go to the AfterUpdate event of the control you want to use as the trigger, and type Form_Current in the code for the sub.
Denis