I wanted to run code on startup/opening an Excel workbook. I tried

Sub Auto_Open()
    Debug.Print "Got here"
    MsgBox "auto_open"
End Sub

It didn’t run when opening the workbook. (Yes, I enabled macros via the Trust Center. I also checked to be sure it was stored in the ThisWorkbook object.) I tried Public and Private variants. It just wouldn’t start. So I changed it to the following, and it runs.

Private Sub Workbook_Open()
    Debug.Print "Got here"
    MsgBox "auto_open"
End Sub