Lisa,

Welcome to the forum!
This code goes in the ThisWorkbook code module:
Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)
    
    Dim UserResponse As VbMsgBoxResult
    
    UserResponse = MsgBox(Title:="Last Updated Date", _
                          Buttons:=vbYesNo + vbQuestion, _
                          Prompt:="Have you updated the Last Updated date if needed?")
    
    If UserResponse = vbNo Then Cancel = True
    
End Sub