I noticed that when I start the Worksheet, the following action is at the top
of my module and does not execute until I click on something on the
Worksheet. In my Worksheet Module I have one SUB().

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Not ActiveSheet.Previous Is Nothing Then
Range("A1").Value = ActiveSheet.Previous.Range("A1").Value
End If
…more code...

I suspect that by the name of the Sub evens are monitored only when an event
is activated.

Now, how do I code a sub() to execute as soon as the Worksheet becomes
active? And where do I put it?