Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Source As Range)
If Source.Address <> "$A$1" Then Exit Sub
On Error Resume Next
Sh.Name = Application.Proper(Format(Source, "mmmm"))
[COLOR="Red"]If Source <> "" And (Err Or Not IsDate(Source)) Then Application.Undo[/COLOR]
End Sub