Private Sub Workbook_Open()
Dim NumSem As Byte
NumSem = DatePart("ww", Date, 2, 2)
For Each sh In Sheets
If sh.Name = "S" & NumSem Then Exit Sub
Next
If Weekday(Date) = vbMonday Then
Sheets("S45").Copy After:=Sheets(Sheets.Count)
Sheets(Sheets.Count).Name = "S" & NumSem
End If
End Sub