Bonjour,
Avec IsNumeric() qui fonctionne très bien chez moi sur ton dernier fichier :
Private Sub Workbook_Open()
Dim Fe As Worksheet
Dim NumSem As Integer
For Each Fe In Worksheets
If IsNumeric(Fe.Name) Then '<--- teste si le nom est numérique !
Select Case Weekday(DateSerial(Year(Date), Month(Date), Fe.Name), vbMonday)
Case 6, 7
Fe.Tab.ColorIndex = Worksheets("Date").Cells(6, 7).Interior.ColorIndex
Case Else
NumSem = Format(DateSerial(Year(Date), Month(Date), Fe.Name), "WW") - Format(DateSerial(Year(Date), Month(Date), 1), "WW")
If NumSem = 0 Then NumSem = 1
Fe.Tab.ColorIndex = Worksheets("Date").Cells(NumSem, 7).Interior.ColorIndex
End Select
If Fe.Name = Day(Date) Then Fe.Tab.ColorIndex = 3
End If
Next Fe
End Sub