Option Explicit
Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Range)
Dim a As Variant
With Sh
a = Switch(.CodeName = "Feuil1", "A2", .CodeName = "Feuil2", "A3", _
.CodeName = "Feuil3", "A4", .CodeName = "Feuil4", "A5")
If IsNull(a) Or Target.Address(0, 0) <> a Or Not IsDate(Target.Value) Then Exit Sub
.PageSetup.RightFooter = "Semaine " & DatePart("ww", Target.Value, 2, 2)
End With
End Sub