"Friday's Child" is a second season episode of the American science fiction television series, Star Trek. It is episode #40, production #32, and was broadcast December 1, 1967. It was written by D.C. Fontana, and directed by Joseph Pevney.

In the episode, the crew of the Enterprise become entangled in a planet's tribal power struggle. Captain Kirk, along with fellow crew members Spock and McCoy, must find a way to bring peace to a race of aliens whose lives revolve around a dichotomy of honesty and savagery. Adding to their difficulty is the presence of the Klingons, whose sole desire is obtaining what they need through guile and/or violence.

The episode's title is derived from a traditional English poem, known as "Monday's Child". The reference is to a line in the poem: "Friday's child is full of woe". Within the episode, the significance is in the unborn child of the planet's murdered ruler, whose prospects of being born depend upon the outcome of Captain Kirk's mission.

On stardate 3497.2, the Federation starship USS Enterprise, under the command of Captain James T. Kirk, arrives at Capella IV to negotiate a mining contract for topeline. The Capellan inhabitants are described by Chief Medical Officer Dr. McCoy as strong, warlike humanoids who average 7 feet in height; and, despite their violent tendencies, the Capellans do have a strong sense of honesty. McCoy is familiar with the Capellans since he spent a few months of his Starfleet career stationed on their world; he speaks their language and knows a great deal about their customs. Kirk is also aware that the Capellans are known to have dealings with the Klingons, which may make the negotiations difficult.

Kirk beams down to the planet with Dr. McCoy, First Officer Spock and a security officer, leaving Chief Engineer Scott in command. He warns him to keep alert for Klingon ships in the area, since they may try to interfere in the talks.

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