Friday Night Death Slot
The Friday Night Death Slot is a reference primarily to network television on Friday nights, specifically the tendency of shows placed in primetime slots on Fridays to end up being quickly cancelled due to low ratings. Many people are busy during that time period on Friday night with family business or socializing or moviegoing, and so ratings tend to suffer. FOX in particular has a history of shows placed in the Friday Night Death Slot being rapidly cancelled, such as Firefly, Boston Public, Dark Angel, Wonderfalls, and John Doe. Other networks have had otherwise popular shows die in the Friday Night slot, most notably CBS with JAG and Joan of Arcadia.
A popular exception to this rule is The X-Files, which actually thrived in the Friday Night Death Slot before moving to its trademark Sunday night slot. NUMB3RS is another show that lasted for several seasons on Fridays. Another show, Ghost Whisperer, spent its entire five-season run on Friday nights.
Whether or not networks put unwanted shows in these slots with the intention of killing them or not is the subject of some debate among cynical television-watchers.