Activate Event
Occurs when a workbook, worksheet, chart sheet, or embedded chart is activated.
Private Sub object_Activate()
object Chart, Workbook, or Worksheet. For information about using events with the Chart object, see Using Events with the Chart Object.
Remarks
When you switch between two windows showing the same workbook, the WindowActivate event occurs, but the Activate event for the workbook doesn't occur.
This event doesn't occur when you create a new window.
Example
This example sorts the range A1:A10 when the worksheet is activated.
Private Sub Worksheet_Activate()
Range("a1:a10").Sort Key1:=Range("a1"), Order:=xlAscending
End Sub