International Dylan Thomas Day (14 May)

Launched at Swansea University in 2015.
Neither birth- nor death-day, the date is that of the first public performance of _Under Milk Wood_ (1954) at the 92nd Street Y's Poetry Center, New York. Actually that was the day it was finished:
"He was up at dawn on 14 May to work on the second half, and he continued writing on the train between Boston and New York....With the performance just 90 minutes away, the "final third of the play was still unorganised and but partially written." The play's producer, Liz Reitell, locked Thomas in a room to continue work on the script, the last few lines of which were handed to the actors as they were preparing to go on stage."
Language point: Why didn't he write in Welsh?
At the 1921 census, Nancy [his sister] and Dylan are noted as speaking both Welsh and English. Their parents were also bilingual in English and Welsh, and Jack Thomas taught Welsh at evening classes.One of their Swansea relations has recalled that, at home, "Both Auntie Florrie and Uncle Jack always spoke Welsh." ...All four aunts and uncles [with whom he spent much time as a boy] spoke Welsh and English....All these relatives were bilingual, and many worshipped at Smyrna chapel in Llangain where the services were always in Welsh, including Sunday School which Thomas sometimes attended. There is also an account of the young Thomas being taught how to swear in Welsh. His schoolboy friends recalled that "It was all Welsh—and the children played in Welsh...he couldn't speak English when he stopped at Fernhill...in all his surroundings, everybody else spoke Welsh..." At the 1921 census, 95% of residents in the two parishes around Fernhill were Welsh speakers.
So, plenty of exposure to it in childhood, and must have spoken it at least a bit.
BUT he was not educated in it:
When he wrote to Stephen Spender in 1952, thanking him for a review of his Collected Poems, he added "Oh, & I forgot. I'm not influenced by Welsh bardic poetry. I can't read Welsh."
also he disliked Welsh nationalism:
Robert Pocock, a friend from the BBC, recalled "I only once heard Dylan express an opinion on Welsh Nationalism. He used three words. Two of them were Welsh Nationalism."
and finally:
Dylan, pronounced ˈ [ˈdəlan] (Dull-an) in Welsh, caused his mother to worry that he might be teased as the "dull one". When he broadcast on Welsh BBC early in his career, he was introduced using this pronunciation. Thomas favoured the Anglicised pronunciation and gave instructions that it should be Dillan /ˈdɪlən/.
https://en.wikipedia.org/wiki/Dylan_Thomas