Cisco :: STP Blocking And Listening Modes

Sometimes when I enable a switchport I see the port goes to Blocking mode first, not sure if it stays in that mode for the 20 secs, I think sometimes is far less than 20 sec. But I have seen too, that when I enable a port it goes directly to Listening mode. When does a switchport goes to the Blocking mode first, mode that all the books I have read say it should go when you enable a port, and when it goes to Listening mode directly?