Private Sub Workbook_Open()
Dim lr As Long
lr = Sheets("log").Cells(Rows.Count, 1).End(xlUp).Row
Sheets("log").Visible = xlVeryHidden 'assuming you do not want people to see the log
Sheets("log").Cells(lr + 1, 1) = "Opened"
Sheets("log").Cells(lr + 1, 2) = Now
End Sub