The song was an important single for Davies and the Kinks, coming in a year of declining commercial fortunes for the band. The song had been intended as an album track but after the relative failure of the previous single "Wonderboy" (which only reached No. 36 in the UK), "Days" was rushed out as a single with an old unreleased track "She's Got Everything" (recorded in February 1966 in the same session as "Dedicated Follower of Fashion") as the B-side. It reached No. 12 on the UK chart, but failed to chart in the U.S. This did not help future releases however as the next four Kinks singles failed to reach the top 30 (two of them failing to chart altogether) in the UK.
The song appeared on an advertisement for the new Volkswagen Golf Cabriolet in 2011 and in the 2013 Comedy film "The Harry Hill Movie" when Harry saves Abu the Hamster from being killed by a crazy vet.
Kirsty MacColl covered "Days" on her 1989 album Kite. It was released as a single and reached No. 12 on the UK singles chart, the same position achieved by The Kinks in 1968. It was re-released in 1995, charting much lower, only reaching No. 42 in the UK. It was one of MacColl's most popular singles.
The single was released in multiple formats. Each featured "Days" and "Happy", and all bar the 7" and cassette featured "Still Life". Both CD formats (the standard case and the Kite-shaped case) had "Please Help Me, I'm Falling", whilst "El Paso" was only available on the 10".
The song was covered by a number of artists at the time including Petula Clark on her 1968 album Petula, The Hillsiders as a B-side to their 1969 single "Kentucky Woman" (RCA 1804) and James Last in an instrumental version on Non-Stop Dancing No. 7 (also 1969).
"Days/This Time Tomorrow" - Ray Davies and Mumford & Sons is featured on Ray Davies solo album See My Friends.
The song was also covered by Flo & Eddie on the album "Flo & Eddie," which was originally released in 1974 and was re-released in 2008 as a double CD with "The Phlorescent Leech and Eddie," which was originally released in 1972.