apologise for asking trivial question.

I have gone through the various instructions in the NG and other pages by
MVPs. still there are many doubts.

this may be a trivial example

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
'comment-some cells are in bold font
Application.EnableEvents = False
If Target.Font.Bold = True Then
Range("a20").Clear
Range("a20") = 12345
Else
Range("a20").Clear
End If
Application.EnableEvents = True
End Sub

why does the change of cells not fire the event procedure.
where is my mistake.
If I want to debug at what line should I introduce a breakpoint.(I learnt it
should be inthe first execuable statement)
when I use the if statment as for example
if target.value 4 etc the event procedure is fired

of course in standard module it works.
thanks and regarads.