Conditional Format If Saturday Or Sunday

I am trying to format a row (B) if B8 = Saturday or Sunday.
The value in B8 is derived from a formula.
I have tried the following but it only works if cell is typed manually.

Private Sub Worksheet_Change(ByVal Target As Range)
On Error GoTo stoppit
Application.EnableEvents = False
With Me.Range("B8")
If .Value = "Saturday" Or .Value = "Sunday" Then
Me.Range("B8:J8").Font.ColorIndex = 2
End If
End With
stoppit:
Application.EnableEvents = True
End
End Sub

I also want to change format back if cell changes to M-F.

Excel 97