The term Friday night death slot refers to the belief that television shows broadcast on Friday nights have a better-than-average chance of being cancelled. In recent years, this has especially been used with regard to the Fox network. Boston Public, Wanda at Large, Firefly, Playing it Straight, Wonderfalls, John Doe and Dark Angel are all examples of Fox series that started on Friday nights and lasted only a few episodes, or were moved to Friday nights, experienced a steep drop in ratings, and were cancelled.

A famous example of a television series brought to a premature death by moving it to Friday nights was the original Star Trek series, which aired on NBC. This occurred because producer Gene Roddenberry lost a fight with Laugh-In producer George Schlatter over the 7:30 p.m. Monday night time slot Roddenberry said he had been promised when the show was renewed after fans deluged NBC with mail in protest.

That would have meant Laugh-In would have had to start a half-hour later, and Schlatter did not see why his show, a ratings smash, had to yield that time to the poorly-rated Star Trek and made no secret of his displeasure. Roddenberry, who never forgave the network for this, made good on a threat to withdraw from personally producing the show, which when combined with the departure of others involved behind the scenes hastened its decline and ensured that there would be no fourth season.

Ironically, the next year NBC began using demographic breakdowns in deciding what shows to air, and discovered that even in the 10 p.m. Friday slot the show had been airing in it nevertheless attracted an audience segment advertisers would have found highly desirable as it consisted mainly of married couples with lots of disposable income.

After cancellation, it went on to decades of syndication and has spawned four additional prime-time television series, an animated series and ten movies across the almost 40 years since the first episode aired in 1966.

In an echo of what happened with the original Star Trek, the sequel series Star Trek: Enterprise was rescheduled from Wednesday to Friday nights on UPN for its fourth season (2004-2005), a move which preceded its cancellation in February of 2005.

Whether networks realize this, and move programs to this day purposefully to justify cancelling programs they do not wish to continue airing, is the subject of much continuing debate and cynicism amongst fans of such programs.

It is likely, though not certain, that the underlying reason for this syndrome has to do with the fact that Friday night is the traditional "first night of the weekend", in the US, at least, and that many people who might otherwise watch television are out partaking in other forms of entertainment because they work a standard work week and need not be up early Saturday morning.

It should also be noted that some shows, such as Dallas, Miami Vice, Falcon Crest and The X Files, have been launched on Friday nights and become successful (although Fox later moved The X Files to Sunday nights).