Do not go gentle into that good night by Dylan Thomas, he begins with "Do not go gentle into that good night". He also uses this as one of two alternating refrains. In this poem, what does the "good night" symbolize?
The "night" is obviously death, but the question is more why it is described as "good".
Dylan Thomas could easily have used a negative adjective such as "dread" or a neutral one such as "dark" to make the poem scan. Instead, he chooses a simple, positive adjective to set up opposing views within the poem. He does not want his father to die, or even to accept death willingly.
But at the same time, he has to admit the opposing view that death is a natural process and often welcomed by people who simply feel that they have had enough. The word "good" is a very abbreviated way of setting up that view; the rest of the poem consists of examples of why he thinks death should be resisted and why even an elderly person should never accept its inevitability.
The final stanza, where he asks his father to simultaneously curse and bless him with his tears, sums up this conflict. His father's "fierce tears" would be the proof of his father's reluctance to die. This would be a blessing in confirming his views, but it would be a curse because it would show his father's grief about his impending death - and it would not prevent him from dying.
Perhaps at this point Thomas realises how selfish his own wishes are.
This is hinted at in a few places in the poem, in particular "rage, rage against the dying of the light," and "Grave men, near death" in the second-to-last stanza, as well as "Though wise men at their end know dark is right."
But the real proof is found here. The first line of the article is:
Thomas' poem for his dying father, exploring the themes of grief and death
With this poem, Dylan Thomas urges his father to not go gently into that good night and to rage, rage against the dying of the light--to survive, to fight death. He's in essence saying "don't die on me."