"Do not go gentle into that good night" is a poem in the form of a villanelle, and the most famous work of Welsh poet Dylan Thomas (1914 -- 1953). Though first published in the journal Botteghe Oscure in 1951, it was written in 1947 when he was in Florence with his family. It was published, along with other stories previously written, as part of his In Country Sleep, And Other Poems in 1952.

Based on this passage, who said do not go gentle into that good night? Let's think fast. Stream of consciousness:
The relevant information to answer the above question is: "Do not go gentle into that good night" is a poem in the form of a villanelle, and the most famous work of Welsh poet Dylan Thomas (1914 -- 1953). So the final answer is Dylan Thomas.