Dylan Thomas completed his villanelle "Do Not Go Gentle into That Good Night" in 1951, a poem in which he urges the dying, specifically his father (who died in 1945), to fight death as long as possible. An interesting aspect of the poem is that Thomas uses the villanelle form, which requires discipline in form and meter, to engage in an anarchic struggle against death.
The first stanza establishes the fierce tone Thomas uses throughout the poem to encourage the dying to fight death:
Do not go gentle into that good night,/Old age should burn and rave at close of day;/Rage, rage against the dying of the light.
Thomas uses a fairly common metaphor for death--the end of day--but he modifies the noun night with the adjective good, an unusual image given his relentless advice to fight against that good night. When we contrast the first line, which uses gentle and good night, with the stanza's third line, which employs the violent "rage, rage" and "dying light," we feel that the difference in tone, as well as the poem's mood, has shifted dramatically.
"Do not go gentle into that good night" is a sentiment that recognizes the subtle attraction death has for those who are sick and dying, as his father was from cancer. Thomas understood that, at some point, those who are dying--and especially those who are suffering--look to death often as a welcome release from their pain: they gently and quietly accept that good night as welcome death. Thomas, however, who is only in his late thirties, is clearly not prepared to lose his father.
Given that Thomas's father was dying from cancer and undoubtedly suffering, Thomas's use of good night is his way of admitting that death has an appeal that, for the suffering person, is hard to resist, but because Thomas, as the child, is simply not prepared emotionally or psychologically to lose his father, he must encourage his father to resist the urge to gently move into that "good night."