Sam explains that "something horrible" could happen at a football match due to hooliganism, further explaining about treating fans "like animals" and "herding them into pens." He's referring to the Hillsborough Disaster of 15 April 1989 in which 96 Liverpool FC fans were crushed to death in a crowd rush.
Events take place on April 21st, 1973. The day Manchester United and Manchester City drew 0-0 at Old Trafford.
Whilst talking to Annie about his out of body experiences, Sam mentions that he "went to see Doctor Who". John Simm, who plays Sam, would later go on to play the Master in the new series of Doctor Who (2005).
From John Simm's website - Originally, the script said Sam Tyler and Gene Hunt were both Manchester City fans but there was no way he (Simm) was having that - he put his foot down. Gene Hunt supported City so Sam Tyler should support United because they were meant to be at odds with each other. Philip Glennister is actually an Arsenal fan, but he has a soft spot for United.