1992: TNG/S05E18 “Cause and Effect“, directed by Jonathan Frakes (Commander Will Riker). The Enterprise becomes stuck in a time loop but the crew retain some memory of previous instances. Guest star Kelsey Grammer is Morgan Bateson.
1967: TOS/S01E26 “Errand Of Mercy” airs for the first time. This episode marked the first appearance of the Klingons. At war with the Klingons, Captain Kirk and Mr. Spock, attempt to sway the incomprehensibly placid population of a planet to resist an invading military occupation.