Karen,
In the ThisWorkbook module you can use a Workbook_Open event handler.
When the workbook opens it will run any code you have there. Put your code to loop through all the worksheets on in ThisWorkbook (use a For Each Worksehet in ThisWorkbook loop) and check your due date against 'Date'. The example assumes your due date is in Range A1. If your due date ends up in a different cell on each worksheet then this will complicate things.
Private Sub Workbook_Open()
Dim wks As Object
For Each wks In ThisWorkbook.Worksheets
If wks.Range("A1") = Date Then
wks.Activate
wks.Range("A1").Select
MsgBox "This project is due today!"
ElseIf wks.Range("A1") < Date Then
wks.Activate
wks.Range("A1").Select
MsgBox "This project is OVERDUE!"
End If
Next wks
End Sub
HTH
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