Looking over the old Christmas listings for Doctor Who at this site, and it's interesting how Susan's departure episode was just 24 hours removed from Christmas Day. DW missed out on what could have been a seminal and emotional moment for anyone watching at the time.
http://ukchristmastv.weebly.com/doctor-who-at-christmas.html
http://ukchristmastv.weebly.com/doctor-who-at-christmas.html