The band's previous album, The Beach Boys Today! (March 1965), represented a departure for the group through its abandonment of themes related to surfing, cars, and teenage love, but it sold below Capitol's expectations. In response, the label pressured the group to produce bigger hits.Summer Days thus returned the band's music to simpler themes.
One outtake from the album's sessions is known as "Sandy" or "Sherry She Needs Me", and was written by Brian Wilson with Russ Titelman alongside "Guess I'm Dumb". "Sherry She Needs Me" was later revisited by the Beach Boys during 1976's Love You sessions. The composition remained dormant until 1998, when it was finally finished by Wilson as "She Says That She Needs Me" for his 1998 Imagination solo album. The Beach Boys' version of "Sherry She Needs Me" was later released in 2013 for the Made in California box set.
Summer Days (And Summer Nights!!) includes Bruce Johnston's first appearance on a Beach Boys album. As Brian Wilson's stage replacement, he was not yet considered an "official" member, but Wilson appreciated Johnston's skills enough to have him contribute vocally and instrumentally on the album. Johnston would often accompany the group on photo shoots, but he was prohibited from having those pictures published on album covers due to a preexisting contract with Columbia Records. Consequently, his image would not grace the jacket of a Beach Boys' album until he appeared on the back cover of Wild Honey in 1967. Along with Johnston, Al Jardine is also missing from the Summer Days cover photo depicting the group on a sailboat, having missed the shoot due to illness.
Summer Days (And Summer Nights!!) proved to be another gold-selling success for the Beach Boys in the US, where it hit number 2 behind The Rolling Stones's Out of Our Heads. Along with 1963's "Surfin' USA" it remains the group's highest-charting studio album in the U.S. The following year, the album would reach number 4 in the UK. The album's lead single, "Help Me, Rhonda", topped the US Billboard Hot 100.
In a 2011 reappraisal, BBC Music observed that the track listing of Summer Days reads "like a Greatest Hits", and felt the album is unfairly disparaged for being "simply loaded with proud pop songs". Comparing to the Beach Boys' later work: "If Pet Sounds is the critics’ favorite, Summer Days is perhaps the people's day at the beach." That same year, the online journal Rocksucker praised the album, ranking it 4th in its list of "Ten Underappreciated Beach Boys LPs", but considers it "an inconsistent collection of which the high points are truly great and the low points ranging from merely good to just-about-passing-muster".
In the early 1970s, as part of Capitol Records' repackage series of their Beach Boys albums, Summer Days (And Summer Nights!!) was retitled California Girls and deleted two tracks: "Amusement Parks, USA" and "I'm Bugged at My Ol' Man". In 1990, the album was reissued paired with The Beach Boys Today!; this package featured extensive liner notes and bonus tracks from that period. In its 2012 reissue, the album received its first true stereo mix.
Schinder, Scott (2007). "The Beach Boys". In Schinder, Scott; Schwartz, Andy. Icons of Rock: An Encyclopedia of the Legends Who Changed Music Forever. Westport, Connecticut: Greenwood Press. ISBN978-0313338458.