On June 2, 1967, The Beatles released the classic Sgt. Pepper’s Lonely Hearts Club Band. according to this day in music.
But this article says it was June 1st (emphasis mine):
Sgt. Pepper’s Lonely Hearts Club Band was an immediate critical and popular sensation – described by fans, music critics and fellow musicians as innovative in every sense.
It was an adventure in concept, sound, song writing, studio technology – not to mention Sir Peter Blake’s eye-popping artwork on the cover.
Released on June 1st, 1967 the album launched the “Summer of Love” – the psychedelia, spirituality, and acid craze that swept the globe.
I’ll go with the article as the accurate one.