Short answers, pulled from the story.
"Rain" was released on the 30th of May 1966 in the United States as the B-side of the "Paperback Writer" single (Capitol 5651), and on the 10th of June in the UK. It was the Beatles' first UK single since the "Day Tripper" / "We Can Work It Out" double A-side in December 1965.
"Rain" was written by John Lennon and credited to the Lennon-McCartney partnership. Lennon said in the early 1970s that he wrote it alone, while McCartney recalled the authorship as "70-30 to John" in his 1997 authorised biography Many Years from Now.
The backwards coda was edited from Lennon's own vocal track and contains the word "sunshine", a drawn-out "rain" from a chorus, and the song's opening line. Lennon claimed he discovered the effect by accident late one night at home, putting the tape on his machine while exhausted. Producer George Martin also claimed credit for the idea, telling Beatles historian Mark Lewisohn in 1987 that he reversed a portion of Lennon's vocal while Lennon was out of the studio.
Ringo Starr has called "Rain" his best recorded drum performance. In 1984, he said: "I think it's the best out of all the records I've ever made. 'Rain' blows me away... I know me and I know my playing... and then there's 'Rain'." Both Ian MacDonald and Rolling Stone described his drumming on the track as "superb".
Authors Mark Hertsgaard and Bob Spitz both identify the 1966 promotional films for "Paperback Writer" and "Rain" as the first examples of music videos. Three films were made for "Rain", directed by Michael Lindsay-Hogg: two filmed at EMI Studios on the 19th of May 1966 and one at Chiswick House in west London on the 20th of May.
Music historian Simon Philo credits "Rain" with marking the birth of British psychedelic rock. The Hindustani gamaka vocal ornamentation Lennon used was later adopted by the Moody Blues on "The Sun Set", the Hollies on "King Midas in Reverse", and Crosby, Stills and Nash on "Guinnevere". Musicologist Walter Everett also identifies the song's closing section as an early example of the "fade-out-fade-in coda" technique, later used by the Beatles on "Strawberry Fields Forever" and by Led Zeppelin on "Thank You".